Depth-Dependent groundwater flow, age, and chemistry in relation to solvent contamination in a production well, Tulare Lake basin, California

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The Tulare Lake basin in the southern Central Valley of California is heavily dependent upon groundwater for drinking water supply but groundwater in this basin is threatened by a wide array of issues. Groundwater aquifers in the alluvial fan sediments derived from the Sierra Nevada on the eastern side of the Tulare Lake basin are highly productive aquifers that are vulnerable to contamination from urban sources.

Location of study area: Tulare Lake Basin

Location of study area: Tulare Lake Basin

Moreover, the urban populations of growing communities of the eastside Tulare Lake basin are highly dependent upon groundwater for water supply. Some of the most prevalent groundwater contaminants derived from urban sources are chlorinated solvents, including tetrachloroethene (PCE). PCE is one of the most commonly detected chlorinated ethenes in groundwater and has been widely used as a solvent in industrial and commercial applications, including dry cleaning. The distribution of sources and transport of chlorinated ethenes in groundwater is challenging to determine because these solvents are Dense Non-Aqueous Phase Liquids (DNAPLs), and therefore, are denser than water. Thus, the source fluids can sink across groundwater flow lines and pool on top of low-permeability sediments, serving as sources of contamination to groundwater that is old enough to otherwise be unaffected by human activities. Understanding where contaminants such as PCE are entering the perforated intervals of long-screened production wells is important for identifying source distribution and contaminant transport processes in the aquifer. An investigation of depth-dependent groundwater flow, age, and chemistry in relation to solvent contamination in a production well in the eastern Tulare Lake basin is highly relevant to local, regional, and national priorities for understanding and mitigating contamination of drinking water supplies.

The goal of this study is (1) Determine the vertical distribution of groundwater flow under ambient and pumping conditions, and the vertical distribution of groundwater age, chemistry, and PCE concentrations under pumping conditions in a contaminated supply well in the eastern Tulare Lake basin. (2) Use depth-dependent data and a local groundwater flow and particle-tracking simulation model to evaluate the hydrogeologic, geochemical, and contamination source processes controlling the vertical distribution of solvent contamination in the supply well.

This work will consist of data collection, data analysis, modeling, and reporting. Data collection will include: 1) collection of wellbore flow data under both un-pumped (ambient) and pumped (dynamic) conditions using an electromagnetic flowmeter (EMFM), and 2) collection of depth-dependent and surface-discharge samples for the analysis of PCE and other chlorinated solvents, major and minor ions, trace elements, nitrate and nitrite, oxygen and hydrogen isotopes, and age tracers. Data analysis of depth-dependent flow and sample data will be used to calculate the concentrations of constituents at selected depths in the aquifer based on the differences in flow between sample depths. Recently published software tools for age tracer and Lumped Parameter Models (LPM’s) will be used to explain the age distribution based on multiple tracers and to estimate the mean age of groundwater samples. Modeling will consist of a radial groundwater flow model using particle tracking to simulate groundwater flow and water quality based on particle tracking. Data collected and analyzed by the USGS and cooperating partners in the study will be compiled, interpreted, and published in a peer-reviewed journal article or USGS publication. The article will co-authored by USGS and cooperating partners on the study.

The proposed study fits within the USGS science strategy direction of conducting a water census of the United States by quantifying and forecasting freshwater resources. The proposed study addresses Water Resources Mission Area and USGS priorities concerning water availability, by contributing to better understanding of geochemical constraints on water availability, and environmental health, by documenting the occurrence of solvents, potential contaminants that can have adverse effects on humans if present in source water used for drinking. Understanding where and how water and contaminants enter production wells will help water managers make informed decisions as to how to cost-effectively respond to groundwater contamination with additional treatment, remediation, well modification, or abandonment.