Isotopes, Microbes, and Turbidity: A Multi‐Tracer Approach to Understanding Recharge Dynamics and Groundwater Contamination in a Basaltic Island Aquifer

Wells designated as groundwater under the direct influence (GUDI) of surface water have caused an ongoing boil‐water advisory afflicting the island of Tutuila, American Samoa for almost a decade. Regulatory testing at these wells found turbidity and indicator bacteria spikes correlated with heavy ra...

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Bibliographic Details
Published in:Ground water monitoring & remediation 2019-02, Vol.39 (1), p.20-35
Main Authors: Shuler, Christopher K., Dulai, Henrietta, DeWees, Randel, Kirs, Marek, Glenn, Craig R., El‐Kadi, Aly I.
Format: Article
Language:English
Subjects:
Aquifers
Bacteria
Casings
Contamination
Dynamics
Flow characteristics
Flow rates
Flow velocity
Groundwater
Groundwater pollution
Groundwater recharge
Isotopes
Maintenance
Microorganisms
Rain
Rainfall
Storm seepage
Surface water
Tracers
Turbidity
Water wells
Well casings
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Summary:Wells designated as groundwater under the direct influence (GUDI) of surface water have caused an ongoing boil‐water advisory afflicting the island of Tutuila, American Samoa for almost a decade. Regulatory testing at these wells found turbidity and indicator bacteria spikes correlated with heavy rainfall events. However, the mechanism of this contamination has, until now, remained unknown. Surface water may reach wells through improperly sealed well casings, or through the aquifer matrix itself. In this study, three independent surface water tracers, turbidity, indicator bacteria, and water isotopes were used to assess recharge timing and determine contamination mechanisms. Results from each method were reasonably consistent, revealing average GUDI well breakthrough times of 37 ± 21 h for turbidity, 18 to 63 h for bacteria, and 1 to 5 days for water isotopes. These times match well with estimated subsurface flow rates through highly permeable aquifer materials. In contrast, where one well casing was found to be compromised, turbidity breakthrough was observed at 3 to 4 h. These results support local management decisions and show repairing or replacing wells will likely result in continued GUDI contamination. Additionally, differences in observed rainfall response for each tracer provide insight into the recharge dynamics and subsurface flow characteristics of this and other highly conductive young‐basaltic aquifers.
ISSN:1069-3629
1745-6592
DOI:10.1111/gwmr.12299