11.1Step 1: State the Problem
WORKSHEET 11: DATA QUALITY OBJECTIVESThe USEPA has developed a seven-step process for establishing Data Quality Objectives (DQOs) to help maximize the likelihood that data collected in the field will be sufficient for the purposes intended (EPA 2006). The following sections implement this seven-step procedure for this study.11.1Step 1: State the ProblemAs discussed in Worksheet 10, one premise of the OU2 risk investigation is that Cl2 and HCl are assumed to be acute constituents of potential concern (COPC), and airborne concentrations are known to occurair in the vicinity of the U.S. Magnesium Plant. is known to beimpacted by releases of Cl2 and HCl. Concentrations of these contaminants in air are expected to vary substantially as a function of distance and direction from the Site, and also as a function of time, depending on both short-term (minutes-hours) and long-term (seasonal) variations in meteorological conditions, as well as variations in release rates from normal plant operations, maintenance activities, and equipment upsets. malfunctions. Available data collected to date, summarized in Worksheet 10 [NOTE. this should include graphs that show C vs t from the original DMA and/or from the gastronics DMA], support the concept that the concentrations of these analytes over time are likely to be characterized by intervals of zero or low concentration with a series of intermittent “spikes” of varying concentration and duration occurring at random times. However, the available ambient air concentration information for these two acute COPCs is too limited to support a risk assessment. The data do not adequately characterize the spatial or temporal patterns of concentration values in air, so additional data are needed to support the characterization of human and ecological exposures to Cl2 and HCl. In preceding RI activities, monitoring systems have been demonstrated that can obtain continuous and representative ambient air concentrations for these target analytes. These systems are to be deployed in Phase 1B to obtain data of sufficient quality and extent to support a risk assessment. However, the existing data are too limited to reliably characterize either the spatial or temporal patterns of concentration values in air, so additional data are needed to support the characterization of human and ecological exposures to Cl2 and HCl.11.2Step 2: Identify the Goal of the StudyThe goal of the study is to collect sufficient data on the concentrations of Cl2 and HCl in air to reliably characterize the pattern of short-term spikes and long-term average exposure concentrations over space and time in order to support reliable evaluation of human and ecological exposures and risks. In effect, the goal of the Phase 1B monitoring program is to deliver sufficient ambient air information with a relatively small number of selected monitoring locations. The focus for Phase 1B monitoring methodology is the acute exposure timescale. Therefore, the monitoring program must be designed to obtain a sequence of short-term ambient air concentrations in candidate areas in which the presence of human and ecological receptors is likely. Longer-term exposures may then be quantified though analysis of the short-term concentration data. 11.3Step 3: Identify Information InputsThe environmental data that are need to be collected during this project to support a reliable characterization of exposure and risk from Cl2 and HCl in air consist of reliable measurements of concentration values both over the short-term and long-term time scales. Because long-term average values may be calculatedderived from an adequate set of reliable and representative short-term values, only the short-term samples requiremeasurements are included in this study collection in the field.Such monitoring data are required at multiple locations around the US Magnesium Plant, with special focus on locations and times where different groups of humans or ecological receptors may tend to be present.Additional information inputs to the development of the DQO and execution of the Phase 1B monitoring program are the data validation and quality assurance measures that will be practiced during and after the field portion of the monitoring program. These methods, as developed in a subsequent Sampling and Analysis Plan (SAP) will follow accepted practices to provide data of adequate quality for risk assessment. Appropriate metrics of magnesium plant production will also be recorded during Phase 1B, to verify that operating levels during Phase 1B monitoring are representative of normal production. Because long-term average values may be calculated from an adequate set of reliable and representative short-term values, only the short-term samples require collection in the field.In addition, data on plant production rates are needed to determine if the time interval when sampling occurred is representative of typical operating conditions.11.4Step 4: Define the Boundaries of the Study11.4.1Spatial BoundaryBased on the expectation that concentrations in air will tend to be highest near the US Magnesium Plant and decrease as a function of distance from the Plant, EPA identified an initial study area described by a circle with a radius of 5 miles, centered on the Plant (EPA 2013a). But within this study area, more detailed spatial boundaries have been identified by USEPA as candidate areas for monitoring to assess exposure risk (USEPA August 2014). Based on qualitative considerations, these sites were viewed by USEPA as representative of the likely locations of human and certain ecological receptors, and that generally represented sufficient spatial coverage within the OU2 overall boundary. Further analysis using a combination of dispersion modeling simulations and ranking tools has shown that five monitoring locations can provide sufficient data for risk-based decisions (ERM May 2015). IIf data collected during the Remedial Investigation indicate that exposures in air may be of concern at distances beyond this initial study area, the study area may be expanded.11.4.2Temporal BoundariesConcentrations of contaminants in air at any specified location within the study area are expected to vary substantially as a function of meteorological conditions, time and plant production patternsrates and operational events, both over the short-term and also seasonally. and also seasonally. TTherefore, it is important that sampling be of sufficient duration to capture the range of values that occur over both the short-term (minutes-hours) and the long-term (seasonally). For purposes of designing the Phase 1B monitoring program, this DQO posesincludes 1-minute instantaneous air concentration readings as the short-term unit of the airborne dataset. This monitoring interval can be reliably performed in field measurements and is a suitable metric for assessing acute exposure. To capture the effects of a full year of seasonal conditions, and the potential emission rate variability, it is proposed that the duration of the Phase 1B sampling program be one year. 11.5Step 5: Develop the Analytic ApproachRisks from exposures to short-term spikes of Cl2 or HCl will be characterized by estimating either the duration (minutes per year) or the frequency (events per year) that a specified receptor will experience. Repeated exposure to short-term spikes is also an aspect of long-term potential risk. an exceedance of a specified Risk-Based Concentration (RBC). Chronic risks will be evaluated by comparing the long-term effective mean exposure concentration to an appropriate chronic reference concentration. The The detailsA summary of how both acute and chronic exposure will be estimated from 1-minute instantaneous air concentration readings is provided below. The specific methodologies of how these data will be used to assess potential human or ecological risks will be described at a later data in the baseline human health and ecological risk assessment technical memoranda for OU 2. these evaluation processes are described below, both for human and ecological receptors.11.5.1Quantification of Hazard for Human Receptors11.5.1.1Risk-Assessment Data for Short-Term ExposuresBased Concentration Values for HumansA number of studies have been performed to characterize the nature and severity of adverse effects following both short-term and long-term inhalation exposures to Cl2 and HCl, both inhumans and in animals. Attachment A summarizes the data and a number of regulatory guidelines that have been established for human exposure to Cl2, and Attachment B summarizes the data and established regulatory guidelines for human exposure to HCl.Available continuous monitoring data surrounding the Plant is limited. But air data obtained during the DMA field work for OU2 does demonstrate the general pattern of concentration events. The variability of local winds was found to result in substantial periods with no measurable acute toxicants concentration present at a monitor with relatively brief, well-spaced events of detectable concentration. In effect, the acute toxicant concentrations pattern at a typical location can be represented as a time-series of short term concentration “spikes”. An example of this concentration data pattern is illustrated in the time-series of 10-minute average chlorine concentrations obtained during the Air Quality DMA in June 2013.EFigure 11.1 – Example DMA Field Data for Acute Toxicant Gas Using the continuous contaminant data collected during the Phase 1B program at OU2 should support a risk assessment that assesses exposure to repeated, infrequent, acute exposures. On this basis, the monitoring data will support evaluation of concentration events that exceed a recognized threshold for reversible acute effects. Acute toxicity thresholds protective of human health and ecological receptors will be presented in the baseline human health and ecological risk assessment technical memoranda for OU 2.11.5.1.25.1.3Characterization of Risks to Humans from Chronic ExposureTo quantify the potential chronic exposure to human receptors, longer-term average concentrations will be calculated using the 1-minute data collected during Phase 1B. The availability of a full-year of 1-minute data at several locations will allow mean average concentrations to be calculated for different timeframes and locations of interest. In constructing this average, an appropriate treatment of the prevalence of 1-minute data that are effectively non-detects is necessary. The bounding cases would be to either treat all non-detects as true zero concentrations, which has some basis in the observation that the plume transport is the only transient mechanism that conveys the gases to a given receptor. A more conservative bounding case would be to assign a level to the non-detects that reflects a background levels that are below the detection limit. Assessment of the treatment of non-detects and the comparison of long-term averages to toxicity thresholds will be addressed in the Baseline Human Health Technical Memorandum. Non-cancer risks to humans from chronic exposures are characterized using the Hazard Quotient(HQ) approach:HQ = EC / RfCwhere:EC = Long-term Exposure Concentration (ppm) RfC = Reference Concentration (ppm)An HQ value ≤ 1 indicates that risk of significant adverse effects is not of concern, while an HQ value > 1 indicates that adverse effects might occur, with the level of concern increasing as the magnitude of the HQ increases.RfC ValuesEPA has not derived a chronic RfC for use in evaluating chronic exposure to Cl2. However, EPA’s Regional Screening Level table identifies the ATSDR chronic MRL (0.00005 ppm) as an appropriate RfC. Consequently, the ATSDR Chronic MRL will be used to evaluate hazard from chronic exposures to Cl2.EPA has derived an RfC for HCl (0.014 ppm). This value will be used in evaluating chronic exposures to HCl.Uncertainties in the use of these toxicity values should be discussed in the uncertainty section of the human health risk assessment.11.5.25.2Characterization of Hazard for Ecological ReceptorsRisks to ecological receptors from inhalation exposures to Cl2 and HCl will be characterized using an approach that is generally similar to that described above for humans. Details are provided below.5.2.1Risk-Based Concentration Values for Ecological ReceptorsA number of studies have been performed to characterize the nature and severity of adverse effects following both short-term and long-term inhalation exposures to Cl2 and HCl in mammals (mainly rodents). Attachment A summarizes the data for Cl2, and Attachment B summarizes the data for HCl. Quantitative exposure-response data in birds are lacking.No existing risk-based concentration values (also referred to as Toxicity Reference Values, or TRVs) were located for exposure of ecological receptors to Cl2 or HCl. In the absence of such established values, EPA is working in collaboration with toxicologists from EPA, Agency partners, and U.S. Magnesium to develop appropriate TRVs for both mammalian and avian receptors. Attention will be focused on values appropriate for protection against moderate (respiratory impairment) and severe (lethal) adverse effects from single and repeated exposure to spikes, and well as protection against effects of chronic (long-term) exposure on growth, reproduction, and survival. After this development process is completed, the resultant ecological TRVs will be provided as part of the ecological risk assessment documents.5.2.2Characterization of Risks to Ecological Receptors from Short-Term ExposuresCharacterization of risks to ecological receptors from exposure to intermittent spikes of Cl2 or HCl will be evaluated by estimating the frequency that exposures above each relevant short-term TRV are likely to occur. For example if the frequency (determined from the data) of the concentration exceeding a 10-minute ecological TRV is 0.00005 (0.005%), and assuming the ecological receptor is present at the site for a total of 4 months per year, the predicted exceedance frequency would be:TRV exceedance frequency = (4 mo/yr x 30 days/mo x 24 hrs/day x 60 min/hr) x (0.00005) / (10 min/event)= 0.5 events per year5.2.3Characterization of Risks to Ecological Receptors from Chronic ExposureRisks to ecological receptors from chronic exposures will be characterized using the HazardQuotient (HQ) approach:HQ = EC / Chronic TRVwhere:EC = Long-tern exposure concentration (ppm)TRV = Toxicity Reference Value for chronic exposure (ppm)Similar to the As was the case for humans, an HQ value ≤ 1 indicates that risk of significant adverse effects is not of concern, while an HQ value > 1 indicates that adverse effects might occur, with the level of concern increasing as the magnitude of the HQ increases.5.3Calculation of Effective Mean Exposure Concentration (EMC)5.3.1Short-term EMC ValuesAs noted above, it is expected that most exposures at the site will be characterized by a series of intermittent spikes of varying concentration and duration. In most cases when exposure concentrations are not constant over time, it is assumed that risk is proportional to the simple average of the concentration values. This assumption is expressed as Haber’s Rule:C x d = Constant effect where:C = exposure concentration d = exposure durationFor example, if Haber’s Rule is true, then exposure to 10 ppm for 10 minutes causes the same effect as exposure to 1 ppm for 100 minutes or exposure to 0.1 ppm for 1,000 minutes.However, for some acute toxicants, Haber’s Rule may not be reliable, and effects are observed to be proportional to the product of time and concentration raised to some power “n” (ten Berge et al. 1986):Cn x d = Constant effectIn cases where C is not constant over the exposure duration d, the metric of exposure is (tenBerge et al. 1986):∫ [C(t)n dt ] = Constant effectBecause C(t) is not a simple function of t, this integral may be approximated as:∑ (C(t)in x ti) = Constant effectwhere ti is the width of sampling observation “i” that has occurred within the time window d. The EMC is defined as a constant concentration of exposure duration d that is toxicologicallyequivalent to a non-constant exposure pattern that occurs over the same exposure duration d:That is:∑ (C(t)in x ti) = EMCn ? d, where d = ∑ tiRearranging this expression to find EMC yields:EMC = [∑ (Cin x ti ) / d ]1/nAssuming that “t” is constant for all sample measurements within a time window, this simplifies to:EMC = [Average of Cn ]1/nBoth NAS (2004) and EPA (2013b) have reviewed the available literature on the concentration- time relationship for Cl2, and have determined that the effect of exposure is not well predicted using Haber’s Rule. Although there is variability between studies, both NAS (2004) and EPA (2013b) concluded that the most appropriate value of the exponent “n” is 2. This value was also selected by CalEPA (2014). Consequently, the EMC for Cl2 for any specified exposure interval of duration d is calculated as:EMCd (Cl2) = [Average of C2 over duration d]1/2For HCl, both NAS (2004) and EPA (2009a) have reviewed the available literature on the concentration-time relationship to determine the value of the exponent “n”. Both NAS (2004) and EPA (2009a) concluded that n = 1. This value was also selected by CalEPA (2014). Consequently, the EMC for HCl for any specified exposure interval of duration d is calculated as the simple average of concentration:EMCd (HCl) = Average of C over duration dThis approach for calculating the EMC for short-term exposures to highly variable exposure concentrations has been reviewed and endorsed by several experts (see Attachment C).5.3.2Chronic EMC ValuesThe strategy for calculating the EMC for long-term or chronic exposures will be to compute the EMC for each day that monitoring has occurred, using the equation above, and then providing the resultant set of one-day EMC values to EPA’s ProUCL software application to compute the mean and the 95% UCL on the mean.The value of long-term EC is calculated from the mean or UCL of the mean EMC by adjusting for intermittent exposure frequency (EPA 2009b):EC(best estimate) = EMC(mean) * ET/24 * EF /365EC(upper bound) = EMC(UCL) * ET/24 * EF /3655.4Counting the Number of RBC or TRV ExceedancesThe probability that the concentration of Cl2 or HCl in air will exceed the AEGL-1 value will be determined from the data simply by counting the number of one-minute values that exceed the AEGL-1, and dividing by the total number of observations. For example, if reliable monitoring data have been obtained at a station once per minute for a year (a total of 525,600 measurements), and the AEGL-1 value was exceeded in 894 of those measurements, the probability would be 894/525,600 = 0.0017 (0.17%).The probability that the EMC in air over a time window of duration “d” will exceed the RBC or TRV for that duration will be estimated by calculating the EMC for each sequential time window of duration d, counting the number of events where the EMC exceeds the RBC or TRV, and dividing by the number of observation windows of duration d:Probability of exceedance (events per time window) = N(exceedance) / N(total)where:N(exceedance) = Number of exceedances counted in the data setN(total) = Total number of sequential time windows of duration d in the data setFor example, assume that a data set of continuous one-minute measurements has been obtained at some specified monitoring station. Assuming that the goal is to calculate the probability (frequency) that the EMC for a 10-minute time window exceeds a 10-minute RBC value, the calculation process is as follows:Step 1. Divide the data into sequential 10-minute blocks. Assuming the data span one full year, there are a total of 60 x 24 x 365 / 10 = 52,560 such time blocksStep 2. Calculate the EMC for each time block, as described aboveStep 3: Count the number of time blocks where the EMC exceeded the RBC. For this example, assume that there were 27 such events.Step 4: Calculate the probability of exceeding the RBCIn this example, the probability of exceeding the 10-minute RBC is: Probability = 27 / 52,560 = 0.0005 (0.05%)5.5Interpreting the Hazard of Short-Term RBC or TRV ExceedancesEPA has not developed national guidelines that establish acceptable limits on the duration or frequency that short-term human RBCs or ecological TRVs may be exceeded. EPA is presently consulting with EPA toxicologists and Agency partners to derive appropriate values, and will also seek and consider input from US Magnesium. When this process is complete, the EPA will establish Exceedance Goals (EGs) for the site, and these will be provided as part of the human health and ecological risk assessment documents. In general, it is anticipated that EGs will become more stringent as the severity of effect increases. In locations where the estimated frequency or duration of exceedances is greater that the relevant EG, action may be needed to reduce exposure and risk to acceptable levels.11.6Step 6: Specify Performance or Acceptance Criteria11.6.1 Decision Error GoalsIn evaluating exposures from Cl2 and HCl in air, two types of decision errors are possible:Type I error: In this case, it is concluded that exposure is within acceptable limits, when in fact the true exposure exceeds acceptable limits.Type II error: In this case, it is concluded that exposure is above acceptable limits, when in fact the true exposure is within acceptable limits.EPA is primarily concerned with minimization of the chances for a Type I error, since an error of this type could result in a failure to address exposures that are of potential human or ecological health concern. In general, EPA has a goal that the probability of making a Type I error should not exceed 5%. Type II errors are of lesser concern, since a Type II error does not result in unacceptable exposures. However, Type II errors may result in the unnecessary expenditure of resources to address exposures that are actually within acceptable limits. Consequently, EPA typically seeks to limit the probability of Type II errors to within a reasonable tolerance. Although there is no standard rule for Type II errors, a value of 20-30% is often identified as a goal. 11.6.2 Characterizing Uncertainty In Short-Term Exceedances ObservationsUsing Phase 1B continuous data, Recall that both concentration event exceedance duration and exceedance frequency are to be calculated based on counts of the number of exceedances concentrations above an accepted threshold that are observed during some sampling duration. Type I and Type II errors may occur because of random statistical variation in the observed counts of concentration events. That is, the observed count may be either higher or lower than the true long-term average count.Sampling at the representative monitoring locations as informed by air dispersion modeling provides reasonable assurance that Type I errors will not occur. These locations can be shown by data adequacy evaluations and possibly by additional modeling during the execution of Phase 1B to show that the selected locations have experienced concentrations that are representative of previous levels as predicted by long-term modeling. Therefore, if during Phase 1B the selected monitoring locations do not exhibit actual concentrations above the Cl2 or HCl exposure risk thresholds, then there is reduced probability that other locations could have experienced such exposures. In order to help minimize the probability of Type II errors, it is desirable to sample for a sufficient duration such that if the monitored frequency of elevated concentration events is below a health-based threshold, then , in cases where the true exceedance rate is below the EG, it can be concluded that the the upper bound of the possible frequency of such events will also be below those thresholds.n the exceedance rate will also meet the relevant EG. Because uncertainty decreases as a function of the total number of observations, longer sampling durations result in decreased statistical uncertainty. Because suitable health-based thresholds EGs are not yet established, quantitative estimates of sampling duration needed to limit Type II errors are not possible. However, once the data are collected,the probability of a Type I and Type II decision error associated with a decision based on either the best estimate or the upper bound may be calculated and presented as part of the uncertainty discussion. 6.2Characterizing Uncertainty in Short-Term ExceedancesThere are several different approaches that may be used to estimate the uncertainty around a measured count. For this effort, the Bayesian uncertainty interval described by Box and Tiao (1973) is selected for use:N(ub) = 0.5 x CHISQ.INV(0.95, 2*N+1)where:N(ub) = 95% Upper Confidence Bound on NCHISQ.INV(p,df) = Inverse chisquare distribution with df degrees of freedom evaluated for probability pGiven N(ub), the upper bound confidence limits on exceedance duration and frequency are calculated as:Upper bound on exceedance duration = N(ub) x (duration of the RBC) Upper bound on exceedance frequency = N(ub) / N(total)In order to help guard against Type I decision errors, EPA will consider both the best estimate and upper bound values of exceedance duration and frequency when assessing exposure to spikes.The equations are as follows:?If the decision is that the exceedance rate based on the best estimate and/or upper bound value of N does meet the EG, the probability of a Type I error is:Prob. of Type I error = CHISQ(EG / k, 2N + 1)where:CHISQ(x,df) = fraction of the cumulative chisquare distribution function with df degrees of freedom that exceeds a value of xk = (ET/24 * EF/365) / (2 * DUR)?If the decision is that the exceedance rate based on the best estimate and/or upper bound value of N does not meet the EG, the probability of a Type II error is:Prob. of Type II error = 1 – CHISQ(EG / k, 2N + 1)6.3Characterizing Uncertainty in Long-Term Average ExposureAs noted above, the long-term average exposure concentration will be derived by first calculating the mean averages EMC for each day that monitoring has occurred, and then using that providing that data set to ProUCL to derive estimates of the longer-term mean and the UCL of the mean. Use of the UCL to estimate exposure concentration automatically limits the probability of a Type I error to 5%. Because the distributional pattern and variance of the data are not known, it is not possible to perform a power calculation to estimate the number of one-day values needed to achieve target tolerances for Type II decision errors. However, because each one-day concentration EMC is derived from many short-term measurements (e.g., 1/minute * 60 min/hr * 24 hrs = 1440), it is expected that the high variance that is likely to be present in the short-term measurements will be greatly reduced in the set of one-day EMC values. Consequently, it is expected that uncertainty in the estimate of long-term average exposure will be small.11.7.0 Step 7: Develop the Plan for Obtaining the DataSeveral different types of continuous sensors that have the ability to measure Cl2 and HCl in air have been evaluated at the Site (ERM 2014, 2015a). Based on field trials of the instruments, monitors available from Gastronics have been identified as the most reliable, and these monitors will be used to collect data at selected monitoring locations within the 5-mile study area. Instantaneous values of both Cl2 and HCl will be measured and recorded once per minute.Details on instrument sensitivity, calibration, and maintenance as well as protocols for data collection, storage and management are provided in this section, and in the attached Gastronics specifications and Operating Manual. 11.7.1Location of Sampling StationsSampling and Analysis MethodsAs noted above, the values and exceedance rates frequency of observed concentrations of Cl2 and HCl are expected to vary as a function of both direction and distance from the US Magnesium Plant. In order to ensure the data are adequate to reliably characterize spatial variability, an initial set of 12 candidate sampling station locations were selected proposed by establishing six equal pie-shaped sectors radiating outward from the plant. The orientation of the sectors was selected so that the first sector (designated “A”) lies with its center line oriented in the predominant downwind direction (southeast) from the Plant. Two monitoring locations were placed within each sector, focusing on areas where human and/or ecological exposures are expected to occur receptors are expected to be present. The 12 candidate locations are shown in Figure 11-1 (Panels A and B).The candidate sampling locations were then grouped into three geographic zones, as follows in Table 11.1:Table 11.1 Roster of Candidate Monitoring Locations by Geographic ZoneZoneCandidate LocationsLakeshore5, 6, 10, 11, 12Central1, 2, 3, 4, 9Upland/Foothills7, 8Within each zone, AERMOD calculations were used to predict the relative likelihood of high concentrations (one-hour values that exceed the 95th percentile of the site-wide data set) occurring within each candidate location, both long term (four year average) and seasonally (by month). Calculations were performed both for releases from the stack and for fugitive releases from the production facility (ERM 2015e). Detailed results are presented in Attachment D, and the results are summarized in Table 11-2. These calculations were used to help select sampling locations within each zone, as discussed below.Lakeshore ZoneBecause the Lakeshore Zone is relatively far from the production facility, it is expected that the plant stack would be the dominant source of Cl2 and HCl concentrations, and that contributions from fugitive releases are likely to be minor. Because of this, comparisons between candidate locations in this zone focused mainly on the AERMOD calculations for stack releases.Of the five candidate locations within the Lakeshore Zone, three (11, 12, and 6) appear to be relatively similar, both with regard to long-term average exceedance frequency predicted average high concentration event frequency (4.0% to 4.6%) and with regard to seasonal variability (a peak in the late winter with a second peak in the late summer). Candidate locations 5 and 10 are also quite similar to each other, but have a relatively lower exceedance frequency of predicted high concentration events (1.7% to 1.9%) than locations 11, 12, and 6, and have only a minor peak in late winter.Based on these results, candidate location 11 is selected to characterize ambient air exposures in the Lakeshore Zone. Use of this station is likely to provide representative data for locations 11, 12 and 6, although the data may tend to be biased somewhat high for locations 5 and 10.Central ZoneBecause the Central Zone includes the area in close proximity around the production facility (especially locations 1-4), it is expected that both plant stack and fugitive sources may be important contributors to measured concentrations of Cl2 and HCl in this zone. Consequently, comparisons between candidate locations in the Central Zone considered the AERMOD calculations for both stack and fugitive releases.When comparisons are based on AERMOD predictions of stack releases, all five candidate locations within the Central Zone (1-4, 9) appear to have similar seasonal patterns, with peaks in the summer months. Three locations (1, 3, and 4) appear to be relatively similar with regard to the long-term average frequency of high concentration events (3.3% to 3.8%), while candidate location 2 is predicted to have substantially higher stack-based frequency of high concentration events (8.5%), and candidate location 6 is intermediate (6%).When comparisons are based on AERMOD predictions of fugitive releases, the pattern is quite different, with peak exceedance frequencies of high concentration events tending to occur in early to late summer, and relatively large differences between expected high concentration exceedances frequencies, with location1 being higher (18%) than locations 2-4 (9-13%) or location 9 (3%).Because the relative contributions of stack and fugitive releases on measured concentrations in the Central Zone is not known, and because each may be important, these AERMOD data do not allow a reliable selection of any single sampling location to be a reasonable representation forthe entire zone. Moreover, the potential for human exposures is more likely in the Central Zone, which makes the data especially important for reliable human health risk characterization.Based on these considerations, it is concluded that characterization of worker exposure in the Central Zone requires reliable measures of concentration at locations 1, 2, and 9. These data will also be valuable in evaluation of potential ecological exposures in the Central Zone, especially location 9.Upland/Foothills ZoneBecause the Upland/Foothills Zone is relatively far from the production facility, it is expected that plant stack releases are likely to be the dominant source of airborne Cl2 and HCl concentrations, and that contributions from fugitive releases are likely to be minor. Because of this, comparisons between candidate locations in this zone focused mainly on the AERMOD calculations for stack releases.Of the two candidate locations within the Upland/Foothills Zone, the long term average exceedance frequency of high concentration events is expected to be somewhat higher at location 7 (4.8%) than location 8 (2.6%). This suggests that location 7 is preferred as a surrogate for this zone. Moreover, location 7 is located in the foothills, in comparison to location 8, which is at the base of the foothills. This is particularly important, because AERMOD calculations suggest that there is are substantial differences between the impact of stack releases on locations in the foothills compared to the base of the foothills during winter months. This is illustrated in Figure 11-2, which plots monthly average relative concentrations along two transects near location 7 that begin in the foothills (nodes a and b) and traverse northeastward onto the uplands (nodes c and d). As shown, there is relatively little difference between these locations in the summer (May – August), but a large difference in other months, especially in January, February, and March. This means that data collected at location 8 might substantially underestimate winter-time exposures to humans and ecological receptors in the foothills.Based on these results, candidate location 7 is selected to characterize exposures in the Upland/Foothills Zone. Use of this station is likely to provide representative data for human and ecological exposures in the foothills, but may tend to overestimate exposures that occur only at the base of the foothills, especially in the winter months.11.7.27.3 Temporal Requirements for Monitoring StationsAvailable data on human land use patterns (ERM 2015b) were used to determine time frames when various human receptors would likely be exposed in each zone. The expected exposure patterns are summarized below in Table 11.2.Table 11.2. Summary of Human Receptor Occupancy of OU2 ZonesZoneReceptorOccupancyLakeshoreShrimp harvestersLand managersOct. - Feb.CentralUS Magnesium workersATI workersHill Bros workers Delivery persons BLM workersYear roundUpland/FoothillsRanchersHuntersRecreational visitorsLand managersOct. – Apr.Sep. – Feb. Mar. – Nov. Year roundAs indicated, human exposures in the Lakeshore Zone are expected to occur mainly in October thru February, but are expected to occur year round for one or more human receptor groups in the Central and Upland/Foothills Zones.Since data acquired during Phase 1B may also be used to assess ecological receptor risk, it is noted that a wide range of avian and mammalian receptors are present at the Site (ERM 2015c, 2015d). The potential for ecological exposures is likely to vary somewhat by species. Representative ecological receptors include the following Table 11.3 (ERM 2015d):Table 11.3. Representative Ecological Receptors in OU2GroupFeeding GuildRepresentative SpeciesAvianTerrestrial Insectivore Aerial insectivore CarnivoreHerbivoreAquatic insectivoreHorned larkTree swallowAm. kestrel, Great horned owlMourning doveSnowy plover, AvocetMammalianHerbivoreTerrestrial InsectivoreCarnivoreKangaroo rat Vagrant shrew BadgerFor avian receptors, the potential for exposures in the spring thru fall (breeding season and fall migration) are of interest, although some avian species only occupy the site in winter, and some are year-round residents. Mammalian receptors are also year-round residents.These occupancy patterns are summarized in Figure 11-3. As shown, the presence of human and/or ecological receptors is expected to occur in all months of the year in each sampling zone. Because some receptors are only present part of the year, and because seasonal exceedance rates may vary substantially in some locations, it is necessary that sampling at each station span one full year in order to capture the range of values that occur over both the short- term (minutes-hours) and the long-term (seasonally).11.7.37.4Sampling Design for Phase 1B Monitoring StationsAs described in Section 11.7.12 of this worksheet, the Phase 1B air monitoring program for OU2 will involve continuous monitoring of acute target gases at five locations for one full year. Each location will house a Gastronics electrochemical air monitor for Cl2 and HCl. One selected location for the full year will also have a co-located, identical Gastronics monitor to provide ongoing assessment of precision. Table 11.4__ shows the manufacturer performance specifications for the Cl2 and HCl sensors. Table 11.4.__. Gastronics True Wireless? Performance SpecificationsSpecification or CriteriaCl2 SensorHCl SensorMeasurement Range0 - 50 ppm0 - 30 ppmMeasurement Resolution< 0.05 ppm< 0.1 ppmOper. Temp. Range- 20 to 40 °C- 20 to 40 °COper. Rel. Humidity Range15 - 95%. non-condensing15 - 95%. non-condensingLong-Term Drift< 10% per 6 months< 3% per monthLinearity of Response< 5% full scale< 5% full scaleResponse Time: To 50% of final value< 20 sec< 30 sec To 90% of final value< 60 sec< 70 secThe operating principles for the Gastronics sensors are similar to those for other electrochemical detectors; the reactive surface of the electrochemical cell. The sensing element consists of a very small “bead” made of an electrically heated platinum wire coil, covered first with a ceramic base such as alumina and then with a final outer coating of palladium or rhodium catalyst dispersed in a substrate of thoria. This type of sensor operates on the principle that when a given gas/air mixture passes over the hot catalyst surface, oxidation/combustion reactions occur and the heat evolved increases the temperature of the “bead”. This in turn alters the resistance across the bead, which can be measured using a standard electrical bridge circuit. The resistance change is then directly related to the gas concentration in the surrounding atmosphere. The Gastronics (dual gas) ambient electrochemical analyzer was selected as the monitoring technology best-suited for long-term continuous monitoring based on the following demonstrated attributes:The response level for the concentration sensors is lower than the lowest acute exposure chlorine and hydrogen chloride exposure criteria;The monitoring system offers high data capture rates and has demonstrated high dataset completeness during normal operation;Hardware is suitable for long term deployment, and will withstand the range of conditions that occur at the locale;Field monitors operate reliably on solar-charged battery power supplies;The time interval between sampled data points is sufficiently short to capture brief concentration events (capable of sub-minute readings);The sensor and monitoring unit accommodate accuracy audits and field calibrations with reference gases to assure data quality; andData are transmitted wirelessly via radio signal to a centralized base station where datasets can be downloaded to a PC and in a convenient format (e.g., Excel spreadsheet) for review and analysis. The Gastronics field monitors transmit digital data in “packets” that are sent from the monitoring stations by radio to a central base unit every several minutes. This operation is controlled by internal code in the base station software, and based on DMA observations with two stations operating at fixed one-minute data intervals, packets are transmitted and received from individual stations every four to seven minutes. Each packet contains the one-minute values for Cl2 and HCl concentration, and battery voltage readings collected since the preceding transmission for that station. Separate data packets are sent and received for each field monitor, and are buffered by the base unit in separate files. Each packet also includes a date/time stamp for the transmitting station. The company indicates that over a dozen such field systems can directly communicate to a single base station. The base station software is configured with an internal data logging function that buffers individual readings from each station in a separate buffer file, so there is no ambiguity about which data is obtained from which station. The collected data is directly queried by a dedicated data logging computer operating the vendor’s Scada software, to create permanent data logs in the form of a .csv file for each individual station. From this computer, data is downloaded periodically to a USB memory device. Procedures for periodic downloading in the field were validated during the Stage 3 DMA activities. These downloading procedures allow verification of the proper capture of data in the data logging computer files. Prior to Phase 1B, ERM will implement phone-modem and communication software to support off-site queries of monitoring status, and to allow data to be downloaded remotely to an office computer. The primary field activity for the US Magnesium monitoring program is periodic quality assurance/quality control (QA/QC). Gastronics recommends that the calibration procedures be performed on a monthly basis. However, for the US Magnesium program the QA audits, function checks, and if needed sensor calibration will be conducted every 7 days of field operation, unless it is shown that monitor performance warrants a longer or shorter QA interval. QA field activities are initiated by challenging each sensor in normal operating mode with certified, single component reference gases (e.g, nominally 10 ppmv Cl2 in nitrogen). This audit provides the “as found” sensor responses for a high concentration gas (approximately 10 ppm Cl2/HCl), a low concentration (approximately 5 ppm Cl2/HCl), and zero air (0.0 ppm Cl2/HCl). ”As found” values of the monitor response are manually recorded in the field, and the reference gas flow will continue at least 2 minutes after than point, to record th audit responses in the electronic record. The primary field quality control (QC) procedure for the Gastronics monitor operation is the zero and span reference gas calibration, which readjusts the internal response curve of the sensor to match zero and user-input span gas concentrations. If a sensor’s “as-found” response was within 10% of the challenge gas concentration, then the recalibration procedure are usually not performed, unless other aspects of the sensor performance is suspect. REFERENCESBox GEP and Tiao GC. 1973. Bayesian Inference in Statistical Analysis. Wiley Online. PrintISBN: 9780471574286. Online ISBN: 9781118033197. DOI: 10.1002/9781118033197CalEPA. 2014. Air Toxicology and Epidemiology. All OEHHA Acute, 8-hour and Chronic Reference Exposure Levels (chRELs) as of January 2014. California Environmental Protection Agency, Office of Environmental Health Hazard Assessment. Available online at: . 2009a. Provisional Advisory Levels (PALs) for Hydrogen Chloride. Final Report. National Homeland Security Research Center, Office of Research and Development, United States Environmental Protection Agency, Cincinnati, Ohio. November, 2009.EPA. 2009b. Risk Assessment Guidance for Superfund. Volume I: Human Health Evaluation Manual (Part F, Supplemental Guidance for Inhalation Risk Assessment). Final. Office of Superfund Remediation and Technology Innovation, Environmental Protection Agency, Washington, DC. EPA-540-R-070-002. OSWER 9285.7-82. January 2009.EPA. 2013a. Phase 1A Remedial Investigation, Sampling and Analysis Plan to Identify Chemicals of Potential Concern in Soils, Sediment, Solid Waste, Water and Air, and Receptor Surveys. Revision 0 for PRI Areas 2 and 8 through 17. US Magnesium NPL Site, Tooele County, Utah. United States Environmental Protection Agency Region VIII. September 2013EPA. 2013b. Provisional Advisory Levels (PALs) for Chlorine (Cl2) (CAS Reg. No. 7782-50-5). Final Report. National Homeland Security Research Center, Office of Research andDevelopment, United States Environmental Protection Agency, Cincinnati, Ohio. November,2013.ERM. 2014. Phase 1A Remedial Investigation, Sampling and Analysis Plan for Operable Unit 2– Ambient Air. U.S. Magnesium RI/FS, Rowley, Utah. Prepared for US Magnesium LLCand USEPA Region 8 by ERM-West. Inc. Revision 1. July 2014.ERM. 2015a. Draft Air Quality PRI (OU-2) Stage 3 Demonstration of Method Applicability – Technical Memorandum, US Magnesium RI/FS, Rowley, Utah. Prepared for US Magnesium LLC by ERM-West, Inc. November 2015. ERM. 2015b. Final Phase 1 Human Exposure Survey Report, RemedialInvestigation/Feasibility Study, US Magnesium Site, Rowley, Utah. Prepared for US Magnesium LLC by ERM-West. Inc. August 2015.ERM. 2015c. Ecological Habitat ReportERM. 2015d. Representative species tech memoERM. 2015e. Phase 1B Air Monitoring Site Selection for US Magnesium OU2. Technical Memorandum to USEPA, Prepared for US Magnesium LLC by ERM-West, Inc. August 2015NAS. 2004. Acute Exposure Guideline Levels for Selected Airborne Chemicals. Volume 4. Subcommittee on Acute Exposure Guideline Levels, Committee on Toxicology, Board on Environmental Studies and Toxicology, National Research Council of the National Academies. The National Academies Press, Washington DC. nap.edu.ten Berge WF, Zwart A, Appelman LM. 1986. Concentration-time mortality response relationship of irritant and systemically acting vapours and gases. J. Hazard. Mater. 13:301–309.Table 11-2. AERMOD Modeling ResultsZoneStationStackFugativeEFPeakEFPeakCentral13.8%May-Jun-Jul17.6%Nov-Dec-Jan28.5%Jun-Jul-Aug12.0%Jun-Jul-Aug33.4%Jul-Aug12.5%Nov-Dec-Jan-Feb43.3%Jun-Jul-Aug9.1%Oct-Nov-Dec96.0%Jul-Aug2.7%Nov-Dec-JanLakeshore114.6%Feb-Mar, Jul-Aug0.8%-- (a)124.1%Feb-Mar, Jul-Oct0.8%-- (a)51.7%Feb0.6%-- (a)101.9%Feb-Mar0.7%-- (a)64.0%Mar-Apr1.7%-- (a)Upland/ Foothills74.8%Sep-Oct-Nov0.2%-- (a)82.6%Jan-Feb0.2%-- (a)EF = Long term average exceedance frequency of the 95th percentile site-wide relative concentration(a) FugativeFugitive emissions are likely of reletivelyrelatively low importance at locations far from the production plant1470660117411500Figure 11-1.Panel A Candidate Air Sampling LocationsSeptember,2015A Sector DivisionCandidate SamplingStation Locations0.5Aerial Imagery:Google Earth 612712015Figure 11-1.PanelB Candidate Air Sampling LocationsSeptember,2015A Sector DivisionCandidate Sampl ngStation Locations250500850646050800N00NI.OOOFHt A.1352550122364500Aerial magery Google EarthJune 2015ENCLOSURE 2 – Revised Worksheet 11: Data Quality Objectes (incl. Attachments A-D)September 17, 2015Figure 11-2. Seasonal Variation Between Foothills and Upland LocationsMonthly Average Relative Concentrations for Transect A1.00.90.80.7228917571120Mean Conc00Mean Conc0.60.50.40.30.20.125831801257300246810121400024681012140.0a b c dJul Aug Sep Oct Nov Dec Jan Feb Mar Apr May JunMonthly Average Relative Concentrations for Transect B1.4a1.2b1.0c228409560960Mean Conc00Mean Conc0.8d0.60.40.22637790698500246810121400024681012140.0Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun000011-23ENCLOSURE 2 – Revised Worksheet 11: Data Quality Objectives (incl. Attachments A-D)September 17, 2015Figure 11-3109601042291000Human and Ecological Occupancy PatternsZone Receptor JanShrimper, Resource managerFebMar Apr May Jun Jul Aug Sept Oct Nov DecLakeshoreBirdsMammalsWinter resident Breeding and migrationWinter residentCentralUpland/ FoothillsWorkers (US Mag, ATI, Hills Bros, BLM) BirdsMammalsRancher Rec visitor HunterBLM Worker Birds MammalsWinter resident Breeding and migration Winter residentWinter resident Breeding and migration Winter resident11-24ATTACHMENT A Toxicity Data for ChlorineATTACHMENT BToxicity Data for Hydrochloric AcidATTACHMENT CCalculation of Effective Mean ConcentrationATTACHMENT DAAERMOD Calculations of Relative Exceedance Frequency atCandidate Monitoring Locations ................
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