6. RDS Monitoring Results

Discussed below are the 2020 monitoring results related to groundwater quality. Included as Appendices are historical summaries of groundwater analytical results for paradioxane, tritium and strontium-90 (Appendix B); and historical summaries of surface water analytical results for paradioxane (Appendix C).

6.1 Groundwater Monitoring

Groundwater monitoring focuses on the fractured shale unit because past monitoring has shown this is the primary zone of contaminant migration.

6.1.1 Laboratory Analysis

Table 6-1 summarizes the paradioxane analytical results from groundwater samples collected in 2020.

6.1.1.1 Paradioxane

• Areal Extent of Plume in Fractured Shale – The extent of areas where concentrations of paradioxane were greater than 50 μg/L (the remedial cleanup standard) in till/fractured shale wells at the time of the annual sampling event in September is depicted on Figure 6-2. See discussion regarding the RDS pumping system operation in Section 8.1.
• Temporal Variations – Temporal variations in concentrations of paradioxane in representative wells upgradient, near and downgradient of the Groundwater Recovery System are shown on Table 6-1. Concentrations of paradioxane detected in 2020 are, on average, lower than the concentrations detected in these wells prior to completion of the RDS source area containment in 2003, as shown below:

• Paradioxane Concentration in Selected Wells (μg/L)

  Well	Location	2000 - 2002 Average	2020 Average	% Reduction
  RDS-10WS	Between disposal area and RDS recovery wells	555	76	86%
  RDS-9WS	Near RDS recovery wells	131	ND	100%
  RDS-11WS	Downgradient of recovery wells	105	48	54%
  RDS-13WS	Downgradient of recovery wells	214	ND	100%

  Exceedances of Groundwater Quality Standards – Table 6-1 summarizes water quality exceedances in 2020. Concentrations of paradioxane exceeded 50 μg/l during one or more sampling events in 8 of the 12 wells sampled, 7 north of Snyder Road and 1 south of Snyder Road (RDS-11WS).

6.1.1.2 Radionuclides

Table 6-2 summarizes the radionuclide analytical results from groundwater samples collected in 2020.

• Temporal Variations – Average concentrations of tritium, strontium-90 and carbon-14 detected in 2020 were generally lower than those detected prior to completion of the RDS source area containment in 2003.
• Exceedances of Groundwater Quality Standards – NYSDEC has not set Ambient Water Quality Standards for radionuclides in groundwater.

6.1.2 Conclusions

Average paradioxane concentrations in groundwater in 2020 were generally below the average concentrations observed prior to installation of remedial measures in 2003 and 2004. Continued pumping and treatment of RDS groundwater is planned for 2021 (see discussion below). The performance of the remedial measures is discussed in detail in section 8 below.

6.2 Surface Water

Surface water samples were collected from four (4) RDS surface water locations and submitted for paradioxane analysis in 2020. Paradioxane was not detected. See Table 6.1.

6.3 Recommendations for 2021

Groundwater and surface water monitoring will continue in 2021 in accordance with the Site Management Plan.  In addition, sampling for low-level 1,4-dioxane will continue as requested by NYSDEC.