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| Titel |
Tracing groundwater recharge in the San Luis Valley, Colorado: Groundwater contamination susceptibility in an agricultural watershed |
| VerfasserIn |
Tanya Patel, Ruth Hindshaw, Michael Singer |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250102722
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| Publikation (Nr.) |
 EGU/EGU2015-2116.pdf |
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| Zusammenfassung |
| Water is a vital resource in any agricultural watershed, yet in the arid western United States
farming practices threaten the quality and availability of groundwater. This is a pressing
concern in the San Luis Valley, southern Colorado, where agriculture comprises 30% of the
local economy, and employs over half the valley population. Although 54 % of the water used
for irrigation is surface water, farmers do not usually apply this water directly to their fields.
Instead, the water is often diverted into pits which recharge the aquifer, and the water is
subsequently pumped during the following irrigation season. The Rio Grande Water
Conservation District recognises that recharge to the unconfined aquifer has been
outpaced by commercial irrigation for at least four decades, resulting in a decline in
groundwater levels. Recycled irrigation water, and leakage from unlined canals
now represent the greatest recharge contribution to the unconfined aquifer in this
region. This makes the shallow groundwater particularly susceptible to agricultural
contamination.
The purpose of this study is to assess groundwater contamination in the unconfined and
upper confined aquifers of the San Luis Valley, which are the most susceptible to
contamination due to their close proximity to the surface. Although concentrations of
potentially harmful contaminants from agricultural runoff are regularly monitored, the large
spatial and temporal fluctuations in values make it difficult to determine long-term trends. We
have analysed δ18O, δ2H and major-ion chemistry of 57 groundwater, stream and
precipitation samples, collected in June 2014, and interpreted them alongside regional stream
flow data and groundwater levels. This will allow us to study the seasonality and locality of
groundwater recharge to provide greater insight into the watershed’s potential for
pollution.
A groundwater vulnerability assessment was performed using the model DRASTIC
(Depth to water, Recharge, Aquifer media, Soil media, Topography, Influence of the vadose
zone and hydraulic Conductivity). Each variable is assigned a weighting and rating, which
provides a quantitative assessment of an area’s pollution potential. Using this method of
investigation, the groundwater vulnerability map produced classifies 5% of the area as
having low pollution potential, 34% as having moderate pollution potential, and
61% as having high pollution potential. The groundwater vulnerability map may
be used to predict the variation in agricultural contaminant concentrations in the
unconfined aquifer. Major ion analyses revealed that nitrate concentrations are highly
variable, varying between 0.435 and 949μM/L, and exceed the EPA maximum
contaminant level at four sites. The spatial variability in nitrate concentrations, as
well as sulphate and phosphate concentrations, is much greater than the differences
predicted by the model. This suggests that this variability is not a result of differences
in the hydrogeology between sites, but instead may be related to individual farm
practices or a result of point sources such as animal waste, septic tanks and sewage
release.
Understanding the impact of commercial irrigation on groundwater quality and
availability is vital for developing effective strategies to stabilise groundwater levels, and
protect the farmers and local population that rely on this water. |
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