Study shows where the water is — and isn’t

Idaho’s latest study on the Lewiston Plateau deep aquifer is providing insight into where development may be sustainable, and where it may not be.

The study, authored by private hydrologist Dale Ralston, is part of legislation passed in 2016 to help inform administrators of the Lewiston Plateau Groundwater Management Area. Such areas are approaching conditions that don’t allow sufficient groundwater to provide a reasonably safe supply for irrigation or other uses at the current or projected rates of withdrawal, according to the Idaho Department of Water Resources.

Ralston studied wells and waterways in the management area and found locations closer to surface water sources like the Snake and Clearwater rivers were more likely to be recharged. Wells drilled in those areas are yielding water at relatively shallow depths, while areas further south can involve drilling more than 1,500 feet.

One place where well drilling gets a little bit iffy is the steadily developing stretch between the eastern Lewiston Orchards and Mann Lake. Nez Perce County Planner Alison Tompkins said the area is becoming a popular place to build ranchettes. But the management area’s well-drilling standards have limited the feasibility of projects that don’t have access to public water systems.

“There are pros and cons, of course,” Tompkins said of such limitations. “The upside is the retention of productive farm ground. The downside is the increased cost of developing rural areas in the management area.”

One major benefit of the study is the ongoing monitoring of the wells, which can help identify the approximate depth of different geological formations where water is or isn’t found, and whether the water is recharged by nearby river systems, Tompkins added.

Lewiston Orchards Irrigation District Financial Manager Jo Ann Cole-Hansen sits on the Water Resources Board. Cole-Hansen said the study is a great tool for both the district and the city of Lewiston as they plan for the future. And it brought good news that supports current efforts to tap new water supplies.

“As far as guiding development, I think it’s telling us that the recharge for the deep water aquifers in the Lewiston area is sustainable,” she said. “They should be able to provide continuous water supplies as development grows east of us on Powers and Grelle.”

The district is planning a network of wells that will eventually replace surface water as its main supply. The wells will all be located in Area A (see map), so Cole-Hansen said there is a high level of confidence they will be productive and not harm adjacent aquifers.

“Only time will tell, but we don’t feel that it will have a significant impact on the city of Lewiston or the (public utility districts), based on this report,” she said.

And even though land to the east and southeast of the Lewiston Orchards might not be the best bet for sustainable wells, LOID plans on developing its distribution and storage system to serve new development in that area, Cole-Hansen added.

The city of Lewiston is also looking at building a major new well to bring much-needed redundancy to its domestic water supply system. Community Development Director Laura Von Tersch said the city will definitely take the report into consideration as it seeks a location for that well.

Ralston collected and analyzed geologic and hydrologic data that focused on the interconnection between aquifers with surface water streams to complete the study. Waterways he examined included the Snake and Clearwater rivers, and Lapwai and Sweetwater creeks, according to the Department of Water Resources.

He wrote that aquifers that recharge only from surface sources like precipitation and irrigation have much less potential for large-scale well development than aquifers that are hydraulically connected to surface water systems.