1. An Analysis of Dissolved Organic Carbon in the Groundwater of Virginia Coastal Plain Aquifers
Elizabeth Keily, Daniel Boehling, Arif M. Sikder, S. Leigh McCallister, & Joseph B. McGee Turner
Introduction
The aquifer system of the Virginia Coastal Plain is broadly divided into pre- and post-Chesapeake Meteor Impact (~35 mya, Figure 2). Presumably the nature of the organic and inorganic phases of the aquifer sediments differs in composition and these variations may be reflected in the quality of groundwater. The goal of the present study is to assess the concentration of dissolved organic carbon (DOC) in the groundwater of the Virginia Coastal Plain aquifers, specifically the Potomac, Aquia, Piney Point, and Yorktown aquifers. We hypothesized that DOC concentrations will vary between aquifers that are pre and post impact, with DOC concentrations greatest pre-impact. DOC, though typically in low concentrations in groundwater, is also a significant factor in the mobilization of metals. Therefore, the presence of DOC in drinking water could have implications for contamination of water quality and public health.
Results
Table 1 indicates DOC concentrations for the 46 wells currently sampled. Groundwater pH ranged from 6.91 to 9.64 and conductivity ranged from -27 to 651 (mV). There was no relationship between DOC concentration and pH or conductivity. Average DOC concentrations by aquifer are as follows:
DOC in the surface aquifer was the lowest at 0.13±0.004 ppm (n=2).
The average DOC from the Yorktown aquifer was the highest at 2.03±2.679 ppm (n=11). A number of groundwater samples from this aquifer seemed abnormally high and will be recollected and analyzed.
The DOC in the water collected from the Piney Point aquifer averaged 0.38±0.267 ppm (n=6).
Groundwater DOC in the Aquia aquifer averaged 0.34±0.123 ppm (n=2).
DOC in groundwater from the oldest aquifer, the Potomac, averaged 0.31±0.183 ppm (n=25).
Figure 1. Map of sampling locations
Conclusions
Sampling is planned to continue in order to increase same size per aquifer, namely surface, Piney Point, and Aquia aquifers.
Ultimately, DOC concentrations will be compared with total and individual dissolved metal concentrations to determine a potential relationship as DOC has previously been shown to facilitate the mobilization of dissolved metals.
Methods
Field procedures:
Water samples were collected from public wells in the four aquifers of the Virginia coastal plain, from approximately Richmond to Mathews on the Middle and Virginia peninsulas (Figure 1). Well locations, provided by the Virginia Department of Health, were sampled from November 2016 to March Water samples were drawn directly from spigots as close to the well as possible. Sample containers were triple rinsed with well water prior to use. DOC samples were collected after filtration though a AcroPak 1500 Capsule 0.2µm filter that was pre-rinsed with 100 L of deionized water to remove any residual carbon. Water samples were run through filtration system for 5 minutes prior to sampling, to purge residual water from other sampling locations. Two 40ml samples were collected at each location and transferred to muffled (500ºC; 5 hours) amber vials and acidified with 80µL of 12 N HCl each. Samples were placed on ice, returned to the lab, and stored in the refrigerator (4ºC) until analyzed. The groundwater pH and conductivity were measured concurrently with a Hanna Instruments pH/ORP & temperature meter.
Laboratory analysis:
DOC concentrations were determined via high temperature combustion with a Shimadzu TOC-5000A total carbon analyzer.
Figure 2. Stratigraphy of Virginia Coastal Plain. (Source: The Virginia Coastal Plain Hydrologic Framework, USGS)
Acknowledgements
We would like to thank Tim Jones for running the DOC analyzes and the numerous undergraduate students that assisted with sample collection.
Table 1. DOC concentrations, pH, and conductivity for 46 sample wells