Guemes Island Environmental Trust:

United States Geological Service (Re)Visits Guemes

United States Geological Service (Re)Visits Guemes

“Nothing new” some people commented, while others enthused “I learned so much,” after the evening lecture by Sue Kahle and Matt Ely, hydro-geologists from the U.S. Geological Survey (USGS). The idea of having a hydrogeology 101 lecture was planned with exactly that goal in mind: to bring people up to the same level of understanding of the complex groundwater issues of our island. Earlier that day some of us, Waterworkers, and other interested persons had met with Sue and Matt to review the ongoing water-well monitoring program and discuss ideas for future work we might do. Here follow some of the issues we talked about.

When the USGS conducted their study of Guemes Island in the early nineties, they did not have the amenities of the Global Positioning System (GPS) to determine location and elevation of the wells with great accuracy. Land elevation at that time was estimated from the location of the well on a geologic contour map. This resulted in estimates with an accuracy of ± 10 ft. Hand-held GPS units are adequate to find the position of the well, but differential GPS units or the old-fashioned rod-and-level surveying from known elevations (benchmarks) are needed for accurate well-head measurements, good enough to construct a flow model for Guemes Island.

A ground-water flow model will give us information about the size of the fresh water “reservoir” under ground. Once such a flow model is in place, one can run the computer model for different conditions such as the effect of a 10-year drought, or a doubling of the population size and find out more about the capacity of our groundwater system under stress. A flow model may also help to identify major recharge areas on the island.

Page 28 of the 1995 USGS report [below] presents an approximate water budget of the island. The quantities for evapotranspiration (evaporation from the surface and through vegetation) and gross recharge are estimated according to certain formulas, while the runoff is the remainder of the total rainfall. The gross recharge is the amount of water, about one-quarter of the total, that enters the groundwater system. Much of this water will meander around and be discharged at various locations, including wetlands and seeps along the shorelines of the island. Only a fraction of the gross recharge will actually reach the water-bearing layers, called aquifers, into which our wells tap.

We asked some questions about the possibility of refining the water budget. Of course, what we would really like to know is how much of the rainwater actually makes it into the aquifer system, and how much we can safely withdraw without depleting the water resource. Refining the budget is a difficult task and it was our impression that Sue and Matt did not consider it the most useful project to pursue.

Wetlands are fascinating and complex structures. They are obviously valuable because they allow water storage and thus slow down run-off over the land surface. Besides that, they can have various functions. Even though the underlying strata may have a very low infiltration rate, recharge of the ground water system may occur over time and during some parts of the year, and then likely mostly around the margins. They may be focal points for contamination of the underlying groundwater system from contaminated runoff. One the other hand, wetlands may also be locations where groundwater is being discharged from the system, particularly when located close to the shoreline.

Many people may not be aware that a water-well monitoring program has been in place for the past ten years. At present we are sampling only five wells. Our goal is to look at the connection between rainfall and SWL, and to detect a possible downward trend in SWL, in case of prolonged drought or over-usage of the water resource.

Sue and Matt consider five to be the minimum number of wells for such a monitoring program and recommend we add a few more for a balanced distribution over the island. They also recommended that we re-visit the old USGS study, measure many of the same wells, and look at changes in water levels, conductivity, and chloride contents over the past 16 years.

We are eager to explore the various ways of contributing to a better understanding of Guemes’ groundwater system. To do so we would welcome valiant volunteers, interesting in helping us with measurements and exploration. If you would like to join Waterworks, please contact Marianne Kooiman at 293-5815 or eyrie@cnw.com

Waterworks Group [8.2.7]


Water, Water Everywhere, But...

An extended series on the freshwater resources of Guemes Island.


From "Hydrogeology and Quality of Ground Water on Guemes Island, Skagit County, Washington." US Geological Survey, 1995

Approximate Water Budget of the Island

An approximate water budget, or distribution of precipitation, for a typical year on the island is presented in table 4. It includes estimates of component values, possible errors associated with each component value, and a likely range of values for each component. This water budget serves as a simple illustration of the fate of precipitation by roughly quantifying the distribution of water in the island's hydrologic system: precipitation, evapotranspiration, recharge, and runoff. Because errors associated with estimation of component values may be considerable, likely ranges of values are presented.

A value for potential evapotranspiration was estimated by using the Thornthwaite energy-budget method as described by Dunne and Leopold (1978), and then actual evapotranspiration was calculated by performing soilmoisture budget calculations as described in Jones (1992). Variables used for these estimations include average monthly precipitation and temperatures from Anacortes; estimated root depths for forest and grassland in the island's dominant soil types; and soil-moisture content. Average annual evapotranspiration was estimated to be 17 in/yr (table 4), or 12-22 in/yr based on estimation errors of as much as 30 percent (H. Bauer, U.S. Geological Survey, written commun., 1994).

The value of 2 inches for runoff (table 4) is a residual; that is, it represents the quantity that remains after evapotranspiration (17 inches) and recharge (6 inches) are subtracted from precipitation (25 inches). Similarly, the value of 5.8 inches for natural discharge from the ground-water system also is a residual; it represents the remainder when the estimated quantity withdrawn by wells (0.2 inches) is subtracted from recharge (6 inches). The large cumulative errors associated with the runoff and natural discharge are evident in their likely ranges of values: 0-4 and 0-12 in/yr, respectively (table 4).

The water budget indicates that a large quantity of precipitation goes to evapotranspiration and that a smaller quantity goes to recharging the island's ground-water system. The smallest quantity of precipitation goes to runoff. Of the water that goes to recharge, only a small fraction goes to pumped wells.

Although ground-water withdrawals account for only a small part of the annual recharge to the system, increased withdrawals could have significant impacts on the system for several reasons. Bredehoeft and others (1982) point out that any additional withdrawal or discharge superimposed on a previously stable system must be balanced by (1) an increase in recharge, (2) a decrease in discharge, (3) a loss of storage within the aquifer (reflected by lowering water levels in wells), or (4) by a combination of these factors. The possibility of increased natural recharge (increased infiltration of precipitation) is unlikely because it would involve major changes in regional weather patterns or increased infiltration rates. Likewise, a decrease in discharge (by pumping wells) is unlikely, because it would necessitate a decrease in water use. The third factor, a loss in fresh ground-water storage, is the one most likely to occur in response to increased withdrawals on the island. Long-term water-level data would be needed, however, in order to verify such changes in storage.

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