Water may be seeping from the land surface to the water table of the High Plains aquifer in western Oklahoma within a few decades rather than the hundreds or even thousands of years it was previously believed to take.

Water users, irrigation and livestock farming in the region may be significantly affected by the discovery. The High Plains aquifer is one of the largest in the US – it extends from South Dakota to Texas and lies under parts of eight states. It provides 20% of the US’ irrigation water.

The aquifer is the only source of drinking water for most of the 150,000 residents of the Oklahoma ‘panhandle’ – a region that includes Cimarron County, Texas County and Beaver County. Beaver County alone uses 500,000 gallons/ day (2,273,000 litres/day).

It was thought that it took hundreds or thousands of years for water and any contaminants in the water to seep down to the water table in the High Plains aquifer. This was because the area receives little rainfall, the water table is on average more than 200 feet (60m) below land surface, and there are layers of naturally-cemented sand and gravel below the soils in much of the region.

A recently published computer model appeared to confirm the established belief that the aquifer is recharged comparatively slowly. But nitrate and tritium concentrations in groundwater samples collected this year by the US Geological Survey (USGS) from the aquifer indicate that rainwater-borne contaminants may reach the water table in parts of the aquifer sooner than previously believed.

It is not yet clear exactly how the rainwater is reaching the water table. The discovery may lead to the implementation of strategies to reduce groundwater contamination in the region, Kathy Peter USGS Oklahoma District Chief told edie. These could include plugging abandoned wells, introducing more stringent standards for the construction of new wells and limiting the application of fertilisers in the vicinity of playa lakes.

USGS Oklahoma District sampled 12 domestic wells in co-operation with the Oklahoma Water Resources Board. Seven of those water samples had tritium concentrations exceeding 2.5 picocuries per liter, indicating recharge from rainfall that fell since 1953, when atmospheric testing of hydrogen bombs began. Tritium is an isotope of hydrogen that is harmless at low concentrations.

Water samples collected for the project are being analysed for ratios of tritium to helium-3 gas or for concentrations of carbon-14 to determine the dates that the groundwater fell as rainfall and started seeping toward the water table.

“We also don’t know what proportion of the contaminants are making it to the water table,” Peter told edie. “That’s the next study.”

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