Impact of geochemical stressors on shallow groundwater quality
Introduction
The deterioration of groundwater quality due to the human activities can lead to adverse effects on human health and ecosystem. Groundwater containing extraneous nutrients and harmful chemicals may also degrade surface water quality by underground streams flowing into the lake. Modelling groundwater contribution to the lake inflow showed that maximum 50% of input to the lakes was groundwater (Lampert and Sommers, 1997). Inflow of nutrients and harmful compounds by groundwater into the lakes may deteriorate water quality and affect sensitive ecosystem within the interaction areas between groundwater and surface water (Burton and Greenberg, 2000).
The Ground Water and Ecosystems Restoration Division (GWERD) of the National Risk Management Research Laboratory (NRMRL) in the US Environmental Protection Agency has installed about seventy monitoring wells surrounding a watershed lake to monitor the groundwater quality seeping into the lake. Groundwater quality along the shoreline of Lake Texoma was monitored for 2 years from the Fall of 1999 to the Fall of 2001 to identify the presence of ecosystem stressors in aquifers that may contribute the quality of shallow groundwater.
Water quality parameters measured were field parameters (dissolved oxygen, redox potential, ferrous iron, pH, conductivity), inorganics (nitrate, nitrite, ammonia, orthophosphate, sulfate, chloride), dissolved methane, dissolved organic carbon (DOC), methyl tert-butyl ether (MTBE), BTEX compounds (benzene, toluene, ethylbenzene, and xylenes), and a suite of metals. Parameter analyses were compared between wells and between 3-month seasonal durations. Land use has an intimate relationship with quality of both surface and ground water. The monitoring wells were classified into three groups, residential area, agricultural area, and oil field area, to understand the effect of land use on groundwater quality. The objective of this study was to investigate the vulnerability of shallow groundwater quality to geochemical ecosystem stressors surrounding the lake. This work provides a better understanding about the parameter relationships in groundwater and gives a perspective how groundwater quality is influenced by geochemical stressors. Better understanding the geochemical stressors of groundwater facilitates more effective resource management, identification of water quality priorities, and a support of strategies that protect and restore water quality.
Section snippets
Study areas
The location of Lake Texoma, study sites surrounding the lake, and monitoring wells in the sites are shown in Fig. 1. The lake is a major component of a watershed system located on the border of Oklahoma and Texas. Lake Texoma has about 93,000 surface acres and is a man-made impoundment of the Red and Washita rivers in southern Oklahoma and northern Texas. The Denison dam was completed in 1947 by The US Army Corps of Engineers. It has become a focus of farming, real estate, and recreation
Monitoring well design
The monitoring wells consisted of 2-in. diameter PVC casing with 4.6 m of screen with 0.0010-in. slot screen size. The casing was installed with 1.5 m of screen above the water table at construction. The well casing was surrounded by medium grained sand to 1.2 m above the top of the screen. Granular bentonite was then added to fill the hole to the surface. The wellhead was finished with a 0.6×0.6 m concrete pad and a locking metal well cover.
Monitoring well sampling
Among the approximately 70 wells installed surround
Field parameters
Fig. 2A shows the range of DO, ferrous iron, and oxidation–reduction (redox) potential values measured for all wells during the study period. Dissolved oxygen levels were from 0.1 to 10 mg l−1 with higher DOs from the same wells occurring in the spring season and with lower DOs in the summer season. Dissolved oxygen in groundwater was in many instances low, which would not contribute directly to adverse water quality. This would also be true for positive mV of redox potential. Water-soluble
Conclusions
The monitoring well assessment did provide an understanding of groundwater quality in the Lake Texoma watershed as related to geochemical stressors. Water-soluble ferrous iron occurred primarily in water samples with a low DO and/or a negative redox potential. Occurrence and level of inorganics in groundwater samples were closely related to the land use and temporal change. Inorganic profiles of samples collected in different areas of the lake showed differences with respect to ferrous iron,
Acknowledgements
The US Environmental Protection Agency through its Office of Research and Development funded and managed the research described here through in-house efforts. It has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. The assistance of Mike Cook of US EPA, Ada, OK in selecting sites is also acknowledged. We thank ManTech Environmental Research Services Corp. for its analytical support.
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