Aquifer recharge estimates completed

Groundwater supplies are replenished at a rate of between 1.44 and 2.49 inches a year on four San Juan County islands, according to a report released by the U.S Geological Survey.

The groundwater recharge estimates were developed in cooperation with San Juan County Health and Community Services.

Groundwater is a major source of drinking water for Lopez, Orcas, San Juan and Shaw islands. Because of the islands’ scenic beauty and low rainfall, they have become popular residential and recreational areas. However, with increasing population and tourism comes increasing concern about having enough clean drinking water. Especially important is to keep the supply free of seawater intrusion.

“A good, accurate estimate of aquifer recharge is essential to assess the availability of ground water and its vulnerability to seawater intrusion,” said Henry Bauer, USGS hydrologist. “Besides the amount and seasonality of rainfall, recharge for the islands is related to the types of geologic deposits, soil cover, and vegetation at the land surface.”

USGS scientists used two different methods to estimate the recharge on the four islands. One method used a water-budget computer program to simulate water conditions and calculate recharge. The other method looked at the chloride in the ground water as well as in precipitation to estimate how much water has passed into the aquifer, bringing atmospheric chloride molecules along for the ride.

The chloride method was used for Lopez Island, estimating about one-fourth as much recharge as the water-budget method.

A previous USGS/San Juan County Health and Community Services study examined concentrations of chloride in ground water samples as an indicator of possible intrusion of seawater on Lopez Island.

Recharge is the water that seeps through the ground to the water table and replenishes the ground water. Like rainfall, recharge can be expressed in terms of inches of water. Here are the estimates from the water-budget method of average annual ground-water recharge for the four islands:

— Lopez Island: The water table rises 2.49 inches per year.

— San Juan Island: 1.99 inches per year.

— Orcas Island: 1.46 inches per year.

— Shaw Island: 1.44 inches per year

The report is titled, “Estimates of Ground-Water Recharge from Precipitation to Glacial-Deposit and Bedrock Aquifers on Lopez, San Juan, Orcas and Shaw Islands, San Juan County, Washington,” by Laura A. Orr, Henry H. Bauer, and J.A. Wayenberg.

Copies of the report are available for viewing at the U.S. Geological Survey, 1201 Pacific Ave., Suite 600, Tacoma, WA 98402.

Copies can be purchased from the U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225-0286. Call (303) 202-4200. It can also be viewed at http://pubs.waterusgs.gov/wri024114.

The USGS, a bureau within the U.S. Department of the Interior, serves the nation by providing reliable scientific information to describe and understand the Earth; minimize loss of life and property from natural disasters; manage water, biological, energy, and mineral resources; and enhance and protect the quality of life.

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