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The U.S. Geological Survey and U.S. Environmental Protection Agency are developing analytical tools to assess the representativeness of groundwater samples from fracturedrock aquifers. As part of this effort, monitoring wells from the Stringfellow Superfund site in Jurupa Valley in Riverside County, California, approximately 50 miles east of Los Angeles, were field tested to collect information to assist in the evaluation and application of in-well flow as computed by the analytical model called the Purge Analyzer Tool, which computes in-well groundwater travel times for simple piston transport of inflowing groundwater from open intervals of a monitoring well to the pump intake and can provide insight into optimal purging parameters (duration, rate, and pump position) needed for the collection of representative groundwater samples. Field testing of wells included hydraulic, chemistry, and dye tracer analysis to investigate travel times in wells under pumping conditions. The Purge Analyzer Tool was able to replicate dye velocities (travel times) for one of three wells that had appreciable inflow from the aquifer but not the other two wells, which are screened in low-permeability sediments and rock, where flow was dominated by borehole storage. A set of criteria was established to help assess the ability to collect representative groundwater chemistry from monitoring wells; criteria included understanding the height of the static well water column and relative exchange rate between the aquifer and the well. 1U.S. Geological Survey. 2APTIM, Inc., under contract to the U.S. Environmental Protection Agency. 3U.S. Environmental Protection Agency. Introduction The U.S. Geological Survey (USGS) and U.S. Environmental Protection Agency (EPA) are developing analytical tools to help evaluate the representativeness of groundwater samples from wells, particularly monitoring wells set in fractured rock. Groundwater chemistry can be affected by physicochemical processes in the wellbore external (ex-situ) to the aquifer. Examples of physicochemical processes include mixing of groundwater from different parts of the aquifer in the well and chemical reactions (ex-situ) from exposure of well water to heating or cooling and to oxygen from the atmosphere (Vroblesky and others, 2007). Currently [2020], most guidelines for collecting representative groundwater samples by pumping from wells involve, in part, achieving stabilization of water-level drawdowns and physicochemical properties of the water (Yeskis and Zavala, 2002). Although stabilization of hydraulics is achievable in many cases, stabilization of physicochemical properties of the water may or may not be achievable based on transient transport and pseudoequilibrium chemical conditions. Therefore, additional tools to help assess the representativeness of samples from a well are needed. For this study, the criteria for determining representativeness of a sample are based on the ability of pumped water to capture recent groundwater inflow from the aquifer to the well. Capture was assessed by computing in-well travel times using the analytical model called the Purge Analyzer Tool (PAT; Harte, 2017; Harte and others, 2019). Field testing was conducted on monitoring wells at the Stringfellow Superfund site, located in Jurupa Valley, in Riverside County, California. 2 Purge Analyzer Tool Application for the Stringfellow Superfund Site in Jurupa Valley, California, in 2017 Purpose and Scope The purpose of this report is to present findings from the “proof of concept” field testing in support of the evaluation and application of the PAT analytical model in replicating in-well flow during pumping of wells set in fractured-rock aquifers. Some specific objectives of this work include (1) identifying the likely time-dependent intervals of capture during purging (pumping) of a well, (2) assessing the heterogeneity of the fractured-rock aquifer and its effect on in-well flow, (3) measurement of in-well groundwater travel times and flow conditions, and (4) developing guidance on sampling and purging requirements needed to collect representative groundwater samples from monitoring wells. This report includes a summary of the field data collected at the Stringfellow Superfund site in support of PAT testing (Harte, 2020), as well as analysis of field data and a discussion of the results. An important component of this work was the evaluation of the existing groundwater monitoring network at the site to collect representative groundwater samples. A subset of wells was chosen for simulations with the PAT and for further evaluation, including detailed field testing. The detailed field testing included borehole geophysical logging, vertical profiles of water chemistry, time-series collection of groundwater samples, and tracking of dispersed dyes in wells to determine pumped water flow conditions and the vertical velocity of well water. The methods used helped formulate important concepts regarding the collection of representative groundwater samples. This report does not contain a detailed explanation of how to run the PAT model; users interested in running the PAT model should consult Harte and others (2019). Site Description The Stringfellow Superfund site in Jurupa Valley in Riverside County, California, approximately 50 miles east of Los Angeles, was used as a disposal site of liquid hazardous waste from 1956 to 1972 (U.S. Army Corps of Engineers, 2016). The original 17-acre property is at the head of Pyrite Creek in Pyrite Canyon in the Jurupa Mountains (fig. 1). During its operation, the site received about 35 million gallons of hazardous waste. The liquid waste was disposed of in unlined disposal ponds and pits in Jurupa Valley. The waste pits and some ponds were capped in the mid-1980s as a part of an interim abatement program by the California Regional Water Quality Control Board Santa Ana Region. Because of waste disposal practices, contaminants were found in the groundwater underlying the site. Dilute plumes of contaminants, particularly trichloroethylene (TCE) and perchlorate, were found to extend southward from the former pit locations as early as 1972. For purposes of remediation, the plume has been divided geographically into four major groundwater zones (fig. 1). The established remediation goal of 5 micrograms per liter (μg/L) for TCE coincides with the EPA maximum contaminant level (MCL). Currently [2020], perchlorate concentrations have no remedial action level but are being compared to the California MCL of 6 μg/L. The subsurface geology consists of unconsolidated alluvium and underlying weathered and unweathered bedrock (granodiorite, quartz diorite, gabbro, and metasediments, including quartzite, schist, and paragneiss). Some wells have open borehole intervals in bedrock, but most of the site wells are completed with a screen and sand pack. Description of the PAT A brief description of the PAT analytical model is provided here; however, readers interested in additional details should consult the tool’s user manual (Harte and others, 2019). The PAT computes in-well groundwater travel times for simple piston transport from open intervals of the well to the pump intake. The model provides insight into optimal purging parameters (time, rate, and pump position) needed for the collection of representative groundwater samples. The PAT can be used to predict (forward mode) purge times required to allow transport of contaminants from screen/open intervals of the well to the pump intake. The PAT can also be used to analyze existing monitoring purge records (reverse mode) to assess the role of well hydraulics on chemical stability of the pumped water. The PAT uses a water budget approach (fig. 2) coupled with the solution of the Dupuit-Thiem equation using a Microsoft Excel VBA code that invokes the embedded iterative solver (Harte, 2017, eqs. 1 and 2; Harte and others, 2019). In this regard, the PAT is a coupled analytical-water budget model. The water budget approach assumes that all flow during pumping is composed of two components, the well storage (excluding aquifer storage) and radial inflow into the well. Radial inflow is further divided into horizontal inflow near the pump intake (the mixing zone [Mz]) and radial inflow outside of the mixing zone that becomes vertical flow. Horizontal inflow into the mixing zone is zero if the pump is situated in the casing. This conceptual model for aquifer well flow can account for different radial inflows into the well from different zones of hydraulic conductivity. Because it assumes minimal vertical-head gradients inside the well, the PAT is applicable to the relatively low purge rates utilized for groundwater sampling.

scientific investigations report

Evaluation and application of the Purge Analyzer Tool (PAT) to determine in-well flow and purge criteria for sampling monitoring wells at the Stringfellow Superfund site in Jurupa Valley, California, in 2017