Trends in concentrations of nonpoint-source contaminants in wells, springs, and streams are related to the history of contamination in groundwater recharge and the age distribution in the groundwater discharge. The age distribution in discharge depends on the groundwater age distribution in the aquifer and the subset of flowpaths that are sampled by the discharge. Groundwater travel times from recharge to discharge are variable; consequently, responses at discharge locations to changing contaminant loading in recharge can include delayed initial responses, dilution of peak concentrations, and prolonged flushing times. These effects are well understood in principle and have important consequences for water resource management (Eberts et al. 2013), but their implications may not be easy to visualize or communicate.
Here we introduce GAMACTT: Groundwater Age Mixtures and Contaminant Trends Tool (Version 1)—an interactive webtool that can be used to explore the effects of basic aquifer properties (saturated thickness, porosity, and recharge rate) and well configurations (tops and bottoms of screened intervals) on groundwater age mixtures in groundwater discharge and on contaminant trends from varying nonpoint-source contaminant input scenarios. The webtool is based on the concept of groundwater stratigraphy whereby changes in contaminant concentrations in recharge may be recorded as a vertical concentration gradient in the aquifer that is related to groundwater age (Cook and Böhlke 2000). The webtool provides a groundwater stratigraphy model that derives age distributions for wells from well construction data by using a partial exponential model (PEM) (Jurgens et al. 2012). By varying the webtool input data (basic aquifer properties, well configuration, contaminant input scenario, and contaminant decay), users can explore concepts such as delays (lag times), dilution factors, and flushing times in hypothetical wells responding to changing contamination of surficial aquifers. In some configurations, results may be applicable to contaminant trends in discharge to surface water bodies like springs and streams. The webtool also illustrates spatial relations between wells and their contributing recharge areas, which are important considerations for point-source and nonpoint-source contaminants.
Important limitations of this webtool include: (1) groundwater model does not include transport through the unsaturated zone; (2) basic aquifer properties do not account for aquifer heterogeneity, flow discontinuities, or dispersion; (3) steady-state model by definition does not provide for transient flow conditions; (4) PEM assumes that the groundwater flow system is not perturbed by pumping or wellbore flow; (5) first-order contaminant degradation model may not be appropriate for some biogeochemical reactions. The webtool is not intended to be used as a prediction tool for real-world applications. Rather, it is presented as an illustrative (educational) tool that enables scientists, hydrogeology professionals, educators, and other interested persons to explore, understand, visualize, and explain various processes affecting how public-supply wells respond to groundwater contamination. The webtool is freely available and can be accessed at http://cida.usgs.gov/gamactt/.
Acknowledgments
The authors are grateful for useful comments from USGS colleague Bruce Lindsey and three anonymous reviewers. Support for this project was provided by the USGS National Water Quality Assessment Program, the USGS Center for Integrated Data Analytics, and the USGS National Research Program.
Disclaimer
Although this software program has been used by the U.S. Geological Survey (USGS), no warranty, expressed or implied, is made by the USGS or the U.S. Government as to the accuracy and functioning of the program and related program material nor shall the fact of distribution constitute any such warranty, and no responsibility is assumed by the USGS in connection therewith. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Supporting Information
Additional Supporting Information may be found in the online version of this article
Screen capture of webtool showing input data, output data, and graphic panels.
Model responses of various well configurations to a simple rectangular contaminant input record, illustrating differences in initial response time (delay), flushing time, and peak concentration.
Model responses of whole aquifer discharge (e.g., fully penetrating well, spring, or stream discharge) to various contaminant input scenarios, with and without degradation.
Description of webtool including example applications (with figures).
References
- Cook PG. Böhlke JK. Determining timescales for groundwater flow and solute transport. In: Cook PG, Herczeg AL, editors. Environmental Tracers in Subsurface Hydrology. Boston, Massachusetts: Kluwer Academic Publishers; 2000. pp. 1–30. [Google Scholar]
- Eberts SM, Thomas MA. Jagucki ML. The quality of our Nation's waters: Factors affecting public-supply-well vulnerability to contamination: Understanding observed water quality and anticipating future water quality. 2013. Reston, Virginia: U.S. Geological Survey Circular 1385.
- Jurgens BC, Böhlke JK. Eberts SM. TracerLPM (Version 1): An Excel® workbook for interpreting groundwater age distributions from environmental tracer data. 2012. U.S. Geological Survey Techniques and Methods Report 4-F3.
Associated Data
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Supplementary Materials
Screen capture of webtool showing input data, output data, and graphic panels.
Model responses of various well configurations to a simple rectangular contaminant input record, illustrating differences in initial response time (delay), flushing time, and peak concentration.
Model responses of whole aquifer discharge (e.g., fully penetrating well, spring, or stream discharge) to various contaminant input scenarios, with and without degradation.
Description of webtool including example applications (with figures).