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Fuel Fabrication Facility - 2020/2021 Annual Groundwater Monitoring Report. Part 2 of 2
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Site:	Westinghouse
Issue date:	09/30/2021
From:	Jeffery Grant; AECOM, Westinghouse
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2020/2021 Annual Groundwater Monitoring Report Appendix D Plume Analytics Technical Memo AECOM

AECOM 7595 Technology Way Denver, CO 80237 aecom.com Project name: CFFF Annual Groundwater Monitoiring Report Project ref: 60641050 To: Jeremy Grant From: Randy Sillan CC: Robin Mikeal Date:

September 27, 2021 Memo

1. Introduction AECOM evaluated the behavior of multiple dissolved phase plumes in groundwater at the Westinghouse Columbia Fuel Fabrication Facility in Hopkins, South Carolina (the Site). The objective of the plume analytics program is to evaluate the behavior over time of constituents of potential concern (COPCs) in groundwater that supports evaluation of plume stability and migration and groundwater monitoring and management strategies.

The methods and results of the plume analytics evaluation are presented in this technical memorandum along with the slide presentation that summarizes the results.

2. Methods 2.1 Plume Analysis Since concentration trends of COPCs at individual wells are not typically representative of overall behavior of COPCs plumes, a whole plume analysis method is used to assess plume-wide behavior and stability. The GroundWater Spatio-Temporal Data Analysis Tool (GWSDAT) is used to evaluate plume behavior by calculating the following plume metrics over time:

The average concentration of a COPC within the dissolved phase plume, The dissolved mass of a COPC in the plume, The area of the dissolved phase plume, and The center of mass of the dissolved phase plume.

For this analysis, a dissolved phase plume contains concentrations of a COPC that exceed a concentration threshold that is typically defined as the cleanup standard or criteria for the COPC in groundwater, which may be site-specific. In general, the maximum contaminant level (MCL) was used as the threshold to define a plume and its extent and calculate its metrics (average concentration, mass, area, and center). For some plume snapshots, the MCL contour is not contained within the monitoring well network, which defines the extent of the spatial domain in which the plume metrics are calculated.

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Memo In other words, the MCL contour extends beyond the domain established by the well network and the plume metrics for a plume defined by the MCL cannot be calculated. In this situation, the threshold value that defines the plume extent is increased until the plume is contained within the spatial domain of the monitoring well network and plume metrics can be calculated.

When evaluating temporal trends in plume metrics as performed in this evaluation, the threshold value was increased until plume metrics could be calculated for each snapshot and provide adequate plume metrics for trend analysis using a consistent threshold value to define the plume. For future iterations of plume analytics, minimum threshold boundaries shall be used until the minimum bounding threshold is equal to the MCL and adequate plume snapshots are available for trend analysis.

The primary tool in GWSDAT for spatial analysis of a plume is a concentration smoother that generates concentration contours of the plume using a nonparametric regression technique known as Penalised Splines (P-Splines). This method evaluates the spatial distribution of monitoring wells and associated concentration data to generate concentration contours that delineate the solute plume for a plume snapshot or sampling event. The concentration contours generated by GWSDAT that define the plume at each time step (snapshot) are used by GWSDAT to calculate the plume metrics (average concentration, dissolved mass, plume area, and plume center of mass).

The plume metrics calculated by GWSDAT are numerical approximations of complex calculations to estimate changes in plume average concentration, dissolved mass, area, and center of mass within the monitoring network over time using multiple sampling events that provide plume snapshots over time. Trends in plume metrics over time approximated by GWSDAT are evaluated using the Mann-Kendall trend test to estimate the statistical significance of trends in the plume metrics over time that can be used to assess plume stability (USEPA, 2009).

Temporal changes in plume average concentration and dissolved mass per foot of aquifer thickness provides an assessment of processes (e.g. source strength, remedial actions, and natural attenuation) that control plume behavior. Temporal changes in the plume center of mass can indicate plume migration or the spatial variability within the plume of processes that change COPC concentrations over time. Likewise, temporal changes in plume area provides an assessment of plume stability and the processes that control distribution of mass within a plume.

Since the objective of performing the spatiotemporal plume analysis is to evaluate the change in plume behavior over time, a consistent monitoring well network that defines the dissolved plume is used during an evaluation time period. In addition, monitoring wells should be included in the analysis that bound and define the extents of the dissolved plume during each monitoring event. The evaluation time period has to include at least four plume snapshots and estimates of plume metrics to evaluate a trend using the Mann-Kendall test. Typically, the confidence in the estimated Mann-Kendall statistic increases as the number of monitoring events increases.

However, the number of monitoring events to include in the plume analytics evaluation should include the most recent events, which provide the best representation of recent and potential future plume behavior.

2.2 Mann-Kendall Trend Test The Mann-Kendall test evaluates whether values tend to increase or decrease over time by analyzing the sign of the difference between later values and all earlier values. The Mann-Kendall test does not assess the magnitude of change. The Mann-Kendall test assumes that a value can always be declared less than, greater than, or equal to another value and that data are independent. The Mann-Kendall test statistic, S, is calculated from the differences in values over time and is a large positive number when later values tend to be larger than earlier values indicating an increasing trend. When S is a large negative number, later values tend to be smaller than earlier values indicating a decreasing trend. When the absolute value of S is small, no trend is indicated.

In addition to the Mann-Kendall statistic, the method calculates the statistical confidence in the Mann-Kendall statistic, S, and the coefficient of variation, COV, for the set of values. Per the methodology presented in Aziz et al. (2003), the following descriptions for trends in values are applied using the specified criteria:

AECOM 2/8

Memo No Trend is applied when the confidence is less than 90% and S is greater than zero (S>0) or the confidence is less than 90%, S is less than or equal to zero (S0), and the coefficient of variation is greater than or equal to one (COV1).

A Stable trend is applied when the confidence is less than 90% and the COV<1.

A Decreasing trend or Probably Decreasing trend is applied when S<0 and the confidence is greater than 95% or 90%, respectively.

An Increasing trend or Probably Increasing trend is applied when S>0 and the confidence is greater than 95% or 90%, respectively.

This methodology for assigning trends in values assumes the statistical significance level, also denoted as alpha, is 0.05 and is the probability of rejecting the null hypothesis when there is no trend. As discussed in Section 2.1, the confidence in an observed trend typically increases with an increase in the number of values being evaluated. To assess stability of dissolved plumes at the Site, trends in the plume metrics (average concentration, mass, and area) over time were evaluated using the Mann-Kendal test and trend descriptions were applied per the methodology in Aziz et al. (2003). For the plume center of mass metric, the distance between the plume center of mass and a specified monitoring well located within the plume was calculated for each plume snapshot and COPC. The trend in this distance was evaluated using the Mann-Kendall test. In most cases the specified monitoring well was the location where the COPC concentration was greatest within the plume during the evaluation period.

2.3 COPCs and Plumes Plume analytics was performed for six COPCs and their associated groundwater plumes:

Tetrachloroethylene (PCE)

Trichloroethylene (TCE)

Fluoride Nitrate Total Uranium Technetium-99 (Tc-99)

The spatiotemporal behavior of the PCE and TCE plumes were evaluated for two separate groundwater zones of the surficial (water table) aquifer: Lower and Upper. Plume metrics and behavior in the Lower and Upper zones were estimated from the concentrations of chlorinated ethenes from four groundwater sampling events between October 2019 and April 2021. Prior to October 2019, the extent of the groundwater sampling network was less and did not delineate the PCE and TCE plumes. The initial threshold concentration for defining the PCE and TCE plumes was 5 micrograms per liter (ug/L). To calculate plume metrics for trend analysis over the four sampling events, threshold concentrations that defined the plume extent for calculation of plume metrics were mostly greater than the MCL and varied based on the COPC and the plume location (Table 1). Based on the distribution of PCE and TCE concentrations greater than 5 ug/L (or a plume-specific threshold concentration) in the Lower and Upper zones and the presence of separate plumes within a groundwater zone, plume metrics were calculated for six plumes as listed in Table 1.

AECOM 3/8

Memo Table 1. Designated PCE and TCE Plumes for Plume Metrics Threshold Sampling Events Groundwater COPC Plume Location Concentration Zone 60 ug/L 4 (Oct 2019 - Apr 2021) Main Plume Upper PCE 6 ug/L 4 (Oct 2019 - Apr 2021) Southern Plume 9 ug/L 4 (Oct 2019 - Apr 2021) Lower Main plume 6 ug/L 4 (Oct 2019 - Apr 2021) Northern Upper TCE 5 ug/L 4 (Oct 2019 - Apr 2021) Southern 39 ug/L 4 (Oct 2019 - Apr 2021) Lower Main plume Although the distribution of TCE in the Upper groundwater zone indicates the presence of an Upper Main plume in the vicinity of the Upper Main PCE plume, trend analysis was not conducted because plume metrics could only be calculated for one (April 2020) of the four sampling events and TCE concentrations within this area were less than 5 ug/L in April 2021.

Concentration data from wells designated as Lower and Upper were combined to evaluate the spatiotemporal behavior of the Fluoride, Nitrate, Uranium, and Tc-99 plumes. The number of groundwater sampling events, threshold concentrations, and separate plumes are summarized in Table 2.

Table 2. List of Designated Plumes for Fluoride, Nitrate, Uranium, and Tc-99.

Threshold Plume COPC Sampling Events Concentration Location Fluoride 4 mg/L 4 (Oct 2019 - Apr 2021) Single plume Nitrate 15 mg/L 4 (Oct 2019 - Apr 2021) Single plume Northern Plume Uranium 30 ug/L 4 (Oct 2019 - Apr 2021)

Southern Plume Tc-99 900 pCi/L 5 (Apr 2019 - Apr 2021) Single plume

3. Results The plume metrics calculated by GWSDAT are provided in Table 3. In most cases, concentration data (and activity data for Tc-99) used to estimate the plume metrics by GWSDAT were limited to data from monitoring wells that delineated the extent of the plume using the threshold concentration (Tables 1 and 2) and included concentration data from within the plume and immediately surrounding the boundaries of the plume. For the fluoride, nitrate, uranium, and Tc-99 plumes; plumes were delineated and plume metrics calculated using data from both the Upper and Lower zones. At locations where there are paired monitoring wells that are screened in either the Upper or Lower zone, the monitoring well with the greater COPC concentration was used to delineate the plume and calculate the plume metrics.

The Mann-Kendall statistic, confidence, COV, and implied trend for the plume metrics are summarized in Table 4. Charts of the plume metrics versus time are provided in the presentation attachment. In summary, most of the delineated plumes are stable or decreasing based on their plume metrics having Mann-Kendall trends that are no trend, stable, or decreasing. For the Main PCE plume in the Lower zone, the dissolved mass and plume area are increasing even though trends in the average concentration in the plume and its center of mass are not statistically significant. For the Southern uranium plume, the Mann-Kendall trends for the plume metrics are AECOM 4/8

Memo increasing indicating that the plume is not stable and increasing. However, the Southern uranium plume and its plume metrics are defined by one monitoring well where concentrations exceed the threshold criteria.

Going forward, the plume analytics evaluation using the methods in this technical memorandum should be performed annually to incorporate new concentrations data that may:

Improve the certainty in the Mann-Kendall statistic and trend, Refine delineation of dissolved phase plumes, and Be used to modify and optimize the long-term groundwater monitoring program.

AECOM 5/8

Memo Table 3. Summary of Plume Metrics for COPC Plumes.

Plume Metric Plume Plume Center COPC Date* Dissolved Center Center Designation Average Area Assumed Distance Mass X-direction Y-direction Concentration1 (ft2) Source2 from (kg/ft)** (ft) (ft)

Source (ft) 10/15/19 70 6.24E-03 10,496 2,023,646.4 745,554.7 34.3 Upper Zone 04/15/20 88 3.63E-02 48,526 2,023,690.2 745,579.2 34.5 W-39 Main Plume 10/15/20 85 4.79E-02 66,155 2,023,699.2 745,585.9 42.5 04/15/21 77 1.71E-02 26,130 2,023,653.4 745,560.6 27.0 10/15/19 23 4.93E-02 255,447 2,024,325.5 744,459.2 160 Upper Zone 04/15/20 19 4.34E-02 267,379 2,024,384.4 744,434.9 104 PCE Southern W-67 10/15/20 16 3.87E-02 277,807 2,024,354.2 744,473.4 132 Plume 04/15/21 17 4.55E-02 314,776 2,024,371.3 744,427.0 119 10/15/19 13 1.74E-01 1,550,173 2,023,499.0 745,522.2 129 Lower Zone 04/15/20 14 1.83E-01 1,550,758 2,023,637.5 745,528.4 154 W-33 Main Plume 10/15/20 14 1.89E-01 1,597,425 2,023,707.7 745,530.5 204 04/15/21 14 1.96E-01 1,693,727 2,023,629.1 745,527.7 148 10/15/19 18 1.41E-03 9,360 2,024,223.7 745,196.0 14.8 Upper Zone 04/15/20 15 1.12E-03 8,817 2,024,226.7 745,191.5 10.8 Northern W-76 10/15/20 14 1.25E-03 10,506 2,024,221.7 745,204.3 23.2 Plume 04/15/21 13 1.11E-03 10,199 2,024,221.8 745,206.9 25.8 10/15/19 6 1.58E-03 28,843 2,024,476.0 744,443.6 18.8 Upper Zone 04/15/20 6 1.74E-03 31,957 2,024,477.8 744,445.8 16.0 TCE Southern W-67 10/15/20 6 1.49E-03 28,917 2,024,480.9 744,444.7 15.7 Plume 04/15/21 6 1.90E-03 35,785 2,024,486.0 744,429.1 30.5 10/15/19 75 8.83E-02 138,766 2,023,862.5 745,655.7 169 Lower Zone 04/15/20 76 7.64E-02 118,270 2,023,879.0 745,675.8 150 W-65 Main Plume 10/15/20 65 7.37E-02 132,680 2,023,835.1 745,640.8 199 04/15/21 68 4.57E-02 79,331 2,023,915.6 745,714.7 114 10/15/19 7 1.42E+01 237,158 2,024,091.4 744,860.9 242 Single 04/15/20 7 1.12E+01 181,519 2,024,067.3 744,833.6 275 Fluoride W-30 Plume 10/15/20 7 1.15E+01 193,860 2,024,091.2 744,867.6 235 04/15/21 7 1.16E+01 205,799 2,024,080.8 744,858.8 247 10/15/19 82 4.36E+02 622,743 2,023,896.0 744,984.4 80.6 Single 04/15/20 77 3.74E+02 573,885 2,023,914.7 744,984.8 88.3 Nitrate W-7A Plume 10/15/20 73 3.77E+02 605,597 2,023,891.1 744,989.7 84.4 04/15/21 72 3.48E+02 569,699 2,023,921.7 744,968.3 78.4 10/15/19 267 1.07E-02 4,704 2,024,219.9 745,370.2 28.1 Northern 04/15/20 115 3.02E-03 3,103 2,024,218.4 745,376.0 22.1 W-55 Plume 10/15/20 213 7.37E-03 4,083 2,024,217.5 745,372.2 25.7 04/15/21 120 3.32E-03 3,244 2,024,218.1 745,374.4 23.6 Uranium 10/15/19 101 2.00E-03 2,342 2,024,340.9 745,165.0 7.97 Southern 04/15/20 102 2.13E-03 2,447 2,024,340.7 745,164.0 7.44 W-77 Plume 10/15/20 104 2.31E-03 2,616 2,024,340.3 745,163.5 7.41 04/15/21 120 2.98E-03 2,925 2,024,340.4 745,164.4 7.92 04/15/19 3,106 2.09E+09 79,111 2,024,052.1 744,850.9 126 10/15/19 2,647 1.86E+09 82,526 2,024,034.8 744,842.5 142 Single Tc-99 04/15/20 2,589 1.66E+09 75,548 2,024,027.8 744,848.0 W-6 141 Plume 10/15/20 2,876 1.83E+09 75,007 2,024,042.5 744,854.1 128 04/15/21 2,671 1.36E+09 60,073 2,024,066.6 744,875.7 97.6 Notes:

Dates are aggregate dates that represent a semiannual sampling event and not the actual date of sample collection.

1 Average concentration units vary by COPC and are mg/L for fluoride and nitrate and ug/L for uranium, PCE, and TCE.

1 Average concentrations unit for Tc-99 is an activity at picocuries per liter, pCi/L.

- Dissolved mass units are kilograms of COPC per foot of aquifer thickness (kg/ft) except for Tc-99, which is total activity of pCi/ft.

The plume center of mass is shown as state plane coordinates in feet.

2 The assumed source location is the location within the plume where the COPC concentration is greatest and is a reference for estimating changes to the distance between the assumed source and the plume center.

AECOM 6/8

Memo Table 4. Summary of Mann-Kendall Trends for Plume Metrics through April 2021.

Mann-Kendall Trends of Plume Metrics COPC Plume Designation Plume Center Average Dissolved Mass2 Area Distance from Concentration1 Source Stable No Trend No Trend Stable Upper Zone Main Plume (S=0, Conf=37.5%, (S=2, Conf=62.5%, (S=2, Conf=62.5%, (S=0, Conf=37.5%,

COV=0.10) COV=0.70) COV=0.65) COV=0.18)

Stable Stable Increasing Stable Upper Zone PCE (S=-4, Conf=83.3%, (S=-2, Conf=62.5%, (S=6, Conf=95.8%, (S=-2, Conf=62.5%,

Southern Plume COV=0.15) COV=0.10) COV=0.10) COV=0.18)

No Trend Increasing Increasing No Trend Lower Zone Main Plume (S=2, Conf=62.5%, (S=6, Conf=95.8%, (S=6, Conf=95.8%, (S=2, Conf=62.5%,

COV=0.02) COV=0.05) COV=0.04) COV=0.20)

Decreasing Stable No Trend No Trend Upper Zone Northern Plume (S=-6, Conf=95.8%, (S=-4, Conf=83.3%, (S=2, Conf=62.5%, (S=4, Conf=83.3%,

COV=0.14) COV=0.11) COV=0.08) COV=0.38 Stable No Trend No Trend Stable Upper Zone TCE (S=-4, Conf=83.3%, (S=2, Conf=62.5%, (S=4, Conf=83.3%, (S=0, Conf=37.5%,

Southern Plume COV=0.03) COV=0.11) COV=0.10) COV=0.34)

Stable Decreasing Stable Stable Lower Zone Main Plume (S=-2, Conf=62.5%, (S=-6, Conf=95.8%, (S=-4, Conf=83.3%, (S=-2, Conf=62.5%,

COV=0.07) COV=0.25) COV=0.23) COV=0.23)

Stable Stable Stable Stable Fluoride Single Plume (S=-4, Conf=83.3%, (S=0, Conf=37.5%, (S=0, Conf=37.5%, (S=0, Conf=37.5%,

COV=0.04) COV=0.12) COV=0.12) COV=0.07)

Decreasing Stable Stable Stable Nitrate Single Plume (S=-6, Conf=95.8%, (S=-4, Conf=83.3%, (S=-4, Conf=83.3%, (S=-2, Conf=62.5%,

COV=0.06) COV=0.10) COV=0.04) COV=0.05)

Stable Stable Stable Stable Northern Plume (S=-2, Conf=62.5%, (S=-2, Conf=62.5%, (S=-2, Conf=62.5%, (S=-2, Conf=62.5%,

COV=0.41) COV=0.60) COV=0.20) COV=0.10)

Uranium Increasing Increasing Increasing Stable Southern Plume (S=6, Conf=95.8%, (S=6, Conf=95.8%, (S=6, Conf=95.8%, (S=-2, Conf=62.5%,

COV=0.08) COV=0.18) COV=0.10) COV=0.04)

Stable Decreasing Decreasing Stable Tc-99 Single Plume (S=-2, Conf=59.2%, (S=-8, Conf=95.8%, (S=-8, Conf=95.8%, (S=-4, Conf=75.8%,

COV=0.08) COV=0.15) COV=0.12) COV=0.14)

Notes:

1 Average concentrations unit for Tc-99 is an activity at picocuries per liter, pCi/L.

2 For Tc-99, total dissolved mass is total activity of pCi/ft.

AECOM 7/8

Memo

4. References Aziz, J.J, M. Ling, H.S. Rifai, C.J. Newell, and J.R. Gonzales. 2003. MAROS: A Decision Support System for Optimizing Monitoring Plans, Groundwater, vol. 41, no. 3, pp. 355-367.

US Environmental Protection Agency. 2009. Statistical Analysis of Groundwater Monitoring Data at RCRA Facilities: Unified Guidance. Washington D.C.: OSWER, EPA 530-R-09-007.

AECOM 8/8

Plume Analytics Summary Westinghouse Columbia Fuel Fabrication Facility October 2019 - April 2021

Plume Analytics - PCE Upper (2019-2021)

Upper Zone Main Upper Zone Main Upper Upper Zone Zone Southern Southern Upper Zone Main Upper Zone Main Upper Upper Zone Zone Southern Southern

Upper Zone Main

PCE Upper Main Plume Graphs (60 ug/L Threshold)

Upper Zone Southern

PCE Upper South Plume Graphs (6 ug/L Threshold)

Plume Analytics - PCE Lower: 9 ug/L (2019-2021)

Inferred Plume Connection Area

PCE Lower Plume Graphs (9 ug/L Threshold)

Plume Analytics - TCE Upper (2019-2021)

Upper Zone Main Upper Zone Main Upper Zone Upper Zone Northern Northern Upper Upper Zone Zone Southern Southern Upper Zone Upper Zone Main Upper Zone Northern Northern Upper Zone Upper Southern Zone Southern

Upper Zone Main Upper Zone Northern Upper Zone Southern

Upper Zone Northern Upper Zone Southern

TCE Upper North Plume Graphs (6 ug/L Threshold)

Upper Zone Main Upper Zone Northern Upper Zone Southern

Upper Zone Northern Upper Zone Southern

TCE Upper South Plume Graphs (5 ug/L Threshold)

Plume Analytics- TCE Lower: 39 ug/L (2019-2021)

TCE Lower Plume Graphs (39 ug/L Threshold)

Plume Analytics - Fluoride: 4 mg/L (2019-2021)

Fluoride Plume Graphs (4 mg/L Threshold)

Plume Analytics - Nitrate: 15 mg/L (2019-2021)

Nitrate Plume Graphs (15 mg/L Threshold)

Plume Analytics - Total Uranium: 30 ug/L (2019-2021)

Northern Northern Plume Plume Southern Southern Plume Plume Northern Plume Northern Plume Southern Southern Plume Plume

Northern Plume Southern Plume

Uranium North Plume Graphs (30 ug/L Threshold)

Northern Plume Southern Plume

Uranium South Plume Graphs (30 ug/L Threshold)

Plume Analytics - Tc-99: 900 pCi/L (2019-2021)

Tc-99 Plume Graphs (900 pCi/L Threshold)

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