Groundwater Age Mixtures & Contaminant Trends Tool (GAMACTT)

GAMACTT (Groundwater Age Mixtures and Contaminant Trends Tool) is 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.

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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 Bohlke 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 pointsource 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.

Please Note: This webtool is designed to guide users through the process of evaluating how groundwater age distributions and changing contaminant inputs affect contaminant trends in wells and other groundwater discharges (Böhlke and others, 2014). As the aquifer model and calculations are idealized and highly simplified, the webtool is intended for educational use and should not be used for predictive purposes. GAMACTT is intended to be used online; if preferred, a downloadable version is also available.: GAMACTT Downloadable Standalone Version (requires Chrome browser)