Project Profile Originally published in October 2003 issue of Water Well Journal. Used with permission of the Water Well Journal. Copyright 2003. National Ground Water Association.

Reducing Seawater Intrusion via Low-tide Pumping

Over an eight-year period, the water association on Camano Island was able to minimize chloride in community wells and protect the island's ground water supply.

By Jerry Purdum and Glenn R. Engel

This article describes efforts to mitigate the effects of seawater intrusion on small water systems on Camano Island, Washington. With demonstrated success of these efforts over an 8 year period, it is hoped that others can benefit from the techniques utilized by these water systems. Some history of the efforts will be explained as well as presentation and analysis of data collected over a 17-year period.

Background: The Seawater Problem

Camano Island, Washington, is a small island located in Puget Sound, surrounded by saltwater. Drinking water is mostly obtained from a narrow or thin aquifer located 80 feet below mean sea level, while the surrounding Puget Sound waters are 300 plus feet deep.

This water source suffers from 'lateral seawater intrusion'. This aquifer was deposited by glacial flows, is generally 5 to 30 feet thick, and consists of fine to coarse sand and small gravel. The sole source of replenishment or recharge is from an average annual rainfall of 25 inches.

In 1994, Sierra Vista Water Association (SVWA) experienced problems with seawater intrusion for the first time. At that time, SVWA was a small two-well water system of less than 100 connections located in the southern panhandle of Camano Island, a section 1 mile wide by 8 miles long. In this area of Camano Island (Figure 1), four adjacent community water systems supply drinking water to an area 3-1/2 miles long by mile wide including 500+ connections. Private wells in the same area also draw water from the same aquifer.

In 1994 the SVWA water system was pumping from two wells with screens 85 feet below MSL into a 30,000 gallon water storage tank at elevation 200 to 210 feet above MSL. Average summer chloride concentrations at that time approached 150 mg/L (parts per million). One inorganic water test revealed a chloride test result of 254 ppm, slightly above the 250 ppm EPA maximum contaminant level (MCL) for this secondary contaminant. The accepted threshold for identifying seawater intrusion is 100 ppm.

The SVWA wells were controlled by a mechanical timer which spread the pumping over a 24-hour period. With two days of water storage available, the timer allowed some mitigation of the very large spikes in demand on the water source, usually in the morning and again in the evening when people return from work.

An extensive collection of data[1], history, research, and consultations with hydrogeologists, representatives from the U.S. Geological Survey, Department of Energy, Department of Health, and water managers/engineers inspired us to study our problem for the purposes of mitigation. A chloride field test kit and static water measurement equipment was acquired to observe chlorides as well as effects of tides on static water elevations. Varied pumping rates, total pumping over time and seasonal variations were observed.

The following observations were made during this investigation:

1. Seawater intrusion increased with total volume pumped as well as with increased rates of pumping.
2. Static well water elevations through tide cycles varied 40 percent of the tide changes. A tide change of 10 feet reflected a change in static well casing water of 4 feet.
3. Chloride concentrations in the well casings were higher at high tides versus low tides. Fresh water floats on salt water and thus the interface between fresh water and seawater goes up and down with the tide. At low-tide this interface point is at its lowest elevation.
4. Drawdown and recovery tests revealed very little drawdown with rapid recovery.
5. Neighboring community water systems were also experiencing seawater intrusion.
6. One neighboring community had chloride tests of 1,400 ppm in 1986 and 548 in 1996, almost 8 percent seawater. (seawater is 18,000 ppm)
7. One community water system was surrounded by private wells that pumped on demand during daylight hours to pressure tanks. This community system tried pumping only at night to avoid pumping simultaneously with its nearby private well owners. This system experienced a significant drop in chloride concentrations.

Mediation Efforts

The observations were presented to the association members at an annual meeting in August 1994. The membership was requested to immediately practice conservation, authorize and finance the addition of a third well, and to increase water storage. These proposals were accepted by the membership.

After the meeting Glenn suggested using a computer to control pumping to take advantage of the observation that the chlorides were apparently lower at low tides. After some discussion it was decided to give the idea a try. Since no commercial product was available at that time to do this type of control, a custom program running on a palmtop computer was developed in conjunction with a custom control circuit.

In January 1995 the computer and control circuitry was installed and test measurements of the system continued. Initial measurements were promising and showed a decline in chlorides over the following year in excess of 50 ppm.