Section J Attachment 6A_Schilling S-1 RA-O_LTM Annual Report Ex Summ 2020.pdf

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Remedial Action - Operation / Long-Term Monitoring Annual Report, 2019-2020 Draft-October 2020 Former Schilling Atlas Missile Site S-1, Bennington, Kansas US Army Corps of Engineers – Kansas City District vi

Executive Summary

This Remedial Action-Operation (RA-O) / Long-term Monitoring (LTM) Annual Report was prepared by the US Army Corps of Engineers – Kansas City District (USACE-KCD) to document activities, present the results, and evaluate the status of the remedy for the first year of post-Decision Document (DD) groundwater monitoring activities conducted during 2019-2020 at the former Schilling Atlas Missile Site S-1 (S-1 Site), a Formerly-Used Defense Site (FUDS) near Bennington, Kansas.

Groundwater at the S-1 Site has been contaminated by chlorinated volatile organic compounds (VOCs) related to historical site operations. The chemicals of concern (COCs) identified in the Decision Document (DD) (USACE-KCD, 2017) are trichloroethene (TCE) and its breakdown product cis-1,2-dichloroethene (cis-1,2-DCE). Additional TCE breakdown products trans-1,2-DCE and vinyl chloride were not selected in the DD as COCs but are monitored as degradation products of TCE.

The Selected Remedy in the final DD is LTM with Alternate Water Supply (USACE-KCD, 2017). The remedial action objective established for the S-1 Site is to mitigate future potential exposure risks to residents from potable use of groundwater containing COCs above the remedial goals (USACE-KCD, 2017).

Prior to 2010, groundwater at the S-1 Site was used as potable water by the current resident. Bottled drinking water was provided in 2010 following detection of TCE above its maximum contaminant level (MCL) in residential well MW-01. As part of the Selected Remedy, a permanent alternate water supply for current on-site residents was constructed in 2019 to replace the TCE-impacted well (MW-01) as the source of domestic water. The current resident still uses groundwater at the S-1 Site for lawn and garden watering.

The final cleanup goals for each COC at the S-1 Site corresponds to its MCLs under the Safe Drinking Water Act (40 CFR 141 Subpart G), which is an Applicable or Relevant and Appropriate Requirement for groundwater (USACE-KCD, 2015):

• TCE: 5 µg/L

• cis-1,2-DCE: 70 µg/L

The other TCE degradation products being monitored and their MCLs are:

• trans-1,2-DCE: 100 µg/L

• Vinyl chloride: 2 µg/L

During 2019-2020, groundwater LTM activities at the S-1 Site were conducted quarterly in accordance with the 2019 RA-O / LTM Plan (USACE-KCD, 2019a). Field activities included synoptic water level measurements, water supply and monitoring well sampling, well inspection and maintenance, investigation-derived waste management, and field operations documentation.

The S-1 Site LTM well network includes a total of 14 wells, 9 shallow wells (MW-01, MW-02, MW-03, MW-04, MW-05, MW-06, MW-07, MW-09, and MW-10S) and 5 deep wells (MW-08, MW-10D, MW-11, MW-12, and MW-13). Synoptic water level measurements indicate water levels per well generally rose over the course of the quarterly measurements during 2019- 2020, but overall were approximately one foot lower than during the 2013 RI. A hydraulic

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Remedial Action - Operation / Long-Term Monitoring Annual Report, 2019-2020 Draft-October 2020 Former Schilling Atlas Missile Site S-1, Bennington, Kansas US Army Corps of Engineers – Kansas City District vii gradient of approximately 0.002 ft/ft was calculated during the RI, which is slightly lower than the average gradient of approximately 0.003 ft/ft calculated during the 2019-2020 events.

During 2019-2020, TCE and cis-1,2-DCE were the only VOCs detected, and all detections were in the shallow wells only. TCE was detected in all 9 shallow wells, and concentrations ranged from 0.57 J µg/L to 13 µg/L, with the maximum concentration of 13 µg/L reported for MW-06 during December 2019. During every quarter, TCE results from MW-02, MW-04, MW-06, and MW-10S exceeded the remedial goal of 5 µg/L.

Degradation product cis-1,2-DCE was detected in 4 of 9 shallow wells (MW-02, MW-04, MW-06, MW-10S) at concentrations ranging from 0.59 J µg/L to 3.6 J µg/L. All cis-1,2-DCE detections were less than the remedial goal of 70 µg/L. Degradation products trans-1,2- DCE and vinyl chloride were not detected in any wells.

In the 4 shallow wells where TCE concentrations historically exceeded the remedial goal of 5 µg/L, MW-02 showed a generally decreasing trend, MW-04 showed a level, slightly increasing trend, MW-06 showed a slightly decreasing trend, and MW-10S showed a slightly increasing trend. Surrounding shallow wells MW-01, MW-03, MW-05, and MW-07 are either nondetect for TCE, or concentrations are less than the remedial goal. Deep wells MW-08, MW-10D, MW-11, MW-12, and MW-13 are all nondetect for TCE.

The extent of TCE contamination has not moved beyond historical boundaries of the well network. While the interpretation of the overall geometry of the TCE plume has changed per quarter, the footprint and concentrations have remained stable.

Although TCE remains present on the S-1 Site at concentrations above the remedial goal, the onsite resident has been provided with an alternate water supply. Annual inspections of the S-1 Site are performed to verify that no additional drinking water wells have been installed onsite nor on the surrounding properties potentially affected by the groundwater plume. Therefore, there are no current risks associated with exposure to groundwater contamination at the S-1 Site and the RAO is being met.

At this time, the current groundwater monitoring network remains adequate and no changes to the groundwater monitoring plan are recommended. COCs TCE and cis-1,2-DCE and non-COC degradation products trans-1,2-DCE and vinyl chloride will continue to be monitored on an annual basis to ensure that the RAO continues to be met. No further sampling from the water treatment building (PWS) is planned. The next annual sampling event will be conducted in approximately May 2021.