Fresh water on Guemes Island: How much do we have? Where does it come from? And what is the quality?
| We don’t yet know the capacity of the aquifers on Guemes nor how much can safely be withdrawn without depleting the reservoir of fresh water. |
Two years ago the Guemes Island Environmental Trust successfully obtained funding
for a United States Geological Survey (USGS) study to look into these questions. A draft report has been under review for six months and the final document is expected out in February 1995.
Because of the interest and importance of this subject, the Guemes Island Environmental Trust has printed in this issue a condensation of the results of the study. This short synopsis can’t come close to communicating all the information in the full, inch-thick USGS report. But we do hope to whet the interest of those who may not be aware of the study and correct any general misconceptions about the nature of the fresh water supply on Guemes Island.
Every summer as the island dries out and the grass goes brown, we need to reflect on the unique character of our fresh water supply, and how we can preserve this resource we all share.
Guemes Ground Water
Where does it come from? Not Mount Baker. How many times have you heard that the source of the fresh water for Guemes is an underground spring from melting glaciers on Mount Baker? Unfortunately, we have no physical connection to any fresh water outside of Guemes. We depend on the rain that falls directly on our island. How much rain do we get?
Guemes is relatively dry. We receive between 22 and 28 inches of rain each year on average. The west side of the island is the driest, and rain measurements taken over a year show that the further east you go on Guemes, the more rainfall there is.
To put our rainfall in a larger perspective, eastern Skagit County receives over four times the annual rainfall we experience on Guemes Island.
What is the geology of the island? Guemes is a flat, complex layer cake of sand, gravel, and clay deposits left by glaciers that passed over this place 10,000 to 250,000 years ago. The layers alternate – impervious and pervious, icing and cake. These glacial deposits lie on top of bedrock that is well below sea level everywhere on the island except for the southeast tip of Guemes, where the bedrock is exposed and forms the familiar high ground. The actual depth to bedrock under the remainder of the island is unknown.
Note: The two illustrations in this article arc Draft Documents from the USGS report. The illustrations are preliminary and subject to revision and may not coiifomi to USGS standards.
What happens to the rain that falls on Guemes? Over two-thirds of the rainfall evaporates on the surface or is taken up by plants. Something less than one-tenth runs off the surface. The remainder, about one-quarter, percolates through this top,layer of the system. What amount of this water flows out the edges at the shoreline cliffs or below the tide line to the sea is unknown. About I / 100 of the total rainfall on Guemes is withdrawn by wells.
How much fresh water is available? The problem is that although we can estimate how much rainfall penetrates the surface layer, normally considered as subsurface recharge, we can’t determine how much ofthis fresh water eventually percolates into the most tapped sand and gravel layer, the Double Bluff aquifer (see fig. 1, pg. 4). About twice as many wells pump from this lower layer as pump from the Vashon aquifer. Since this lower aquifer is largely capped by up to 200 feet of clay in the Whidbey confining unit, the actual amount of fresh water that recharges the Double Bluff aquifer, Guemes Island’s largest aquifer, remains unknown.
We do know that most of the Double Bluff aquifer is below sea level. This is illustrated on the Model of the Ground Water System below. The top of the freshwater in this lower aquifer was calculated by the USGS by measuring the distance from the ground surface to the water level in each well. The top of the fresh water is also called the potentiometric surface and on Guemes is generally 0 to 15 feet above sea level. To get a sense of how little above sea level the potentiometric surface is, turn to the eight-inch high outline of Guemes that’s on the front page of this newsletter and lay it flat on a table. Imagine that the table surface is sea level. The top of the paper would represent the top of potentiometric surface in the Double Bluff aquifer. In other words, in scale, the distance between sea level and the top of the fresh water in would be about the thickness of a sheet of paper.
What about salt water in our wells? Here’s the big concern. A number of wells on Guemes are pumping an increasing amount of salt mixed with fresh water. As you can see from the map, most of the problem has been charted near the shorelines. The state standard threshold for concern is when the water contains more than 100 milligrams per liter of chloride from sea water. Some of the wells measured on Guemes were found to have more than 500 milligrams per liter of chloride.
While there is documentation that the amount of salt water intrusion has been increasing, and since other factors have remained constant, the problem is related to the amount of water withdrawn from wells. However, the USGS study did not completely explain the mechanism for salt water intrusion. We don’t know, for example, if the problem is only related to the shoreline well withdrawal, or whether water withdrawn from a more central point on the island would also pull sea water inland.
What’s next? We don’t yet know the capacity of the aquifers on Guemes nor how much can safely be withdrawn without depleting the reservoir of fresh water. But we now have a tremendous base of data on the hydrogeology of the island – some of the most complete hydrogeological information ever compiled in this state for a specific locale. More importantly, we know what we have to do to get answers to questions about capacity. A detailed program to get these answers is spelled out in another article in this newsletter.
The Guemes Island Environmental Trust will continue to work to acquire more information on the nature of the ground water system. Expanding our knowledge ofthe ground water system is in everyone’s interest.
– Tim Rosenhan, Fall 1994
Water: Where Do We Go From Here?
Editor’s note: John Oldow is a Professor of Geology at Rice University in Houston, Texas. John grew up in Anacortes and graduated from Anacortes High School in 1968. He owns a home on the southwestern corner of Guemes Island and has taken a personal and professional interest in the study o fthe ground water on the island.
During the time which has elapsed since John submitted the following article, he and several of his colleagues at Rice University have formulated a much broader water resource investigation proposal than that outlined below. The study will build on the large source of information provided by the recent United States Geological Survey (USGS) Ground Water Study of Guemes Island. At least three principal investigators will participate. Their combined expertise covers general geology, potential field analysis, sedimentology, and seismology. In addition, a Ph.D. candidate intends to complete his doctoral study with the Guemes Island investigation anddata analysis, under John’s supervision. The initial stage of the investigation will be supported partly through volunteer efforts; later stages will be supported by Rice University, the National Science Foundation, and other grants.
On November I of this year, John presented his research proposal to the Skagit County Board of Health, a board composed of the county commissioners. The commissioners clearly were very interested and impressed, and offered their written support for the project if such would be helpful at any stage of the investigation. Already Sue Kahle, project leader ofthe USGS study, has written us the following with respect to the proposal: “It is an excellent and rare opportunity to further define the hydrogeology of the island employing state-of-the art methodologies and many new data points.”
Introduction
As pointed out in the accompanying article by Tim Rosenhan, the United States Geological Survey (USGS) hydrogeology study contributed a new and sorely needed geologic synthesis of the aquifer systems on Guemes and also acquired an impressive amount of data concerning rainfall, potential recharge rates, and the depths of and fluctuations in well water levels across the island. Although the study represents an excellent start given available time, funds, and equipment, several vital questions could not be addressed by the USGS personnel. Just before my return to Houston this past summer, I was asked to write a brief summary of the next steps that are needed to assess the water resource on Guemes Island. I have worked with other members of our community in an attempt to focus the results ofthe study on two ofthe island’s fundamental questions: how much water exists, and where is it located?
The need for a clear understanding of the water resource of the island has been dramatically demonstrated by the severe salt water intrusion of the water system at Potlatch Beach on the west side of the island. During this past summer, I became aware of the severity of the Potlatch problem and, like many of you, heard rumors of various proposed solutions ranging from drilling additional wells to piping water to Guemes from Anaeortes. The Potlatch problem has served to focus the attention of Guemes Islanders on a primary concern that involves us all, for we may all face the same future if prudent steps are not taken both in the short term and in the long term.
Nature of the Problem
To make sound decisions concerning the water resource on Guemes Island, more information is needed. Specifically, we need a better understanding of 1) the volume of fresh water in the aquifers tapped by the wells on the island, and 2) a clear view of lateral flow or transfer of water in the aquifers between different parts of the island.
Typically, water depth information is used to determine the potentiometric surface (hydrostatic pressure level) of the water contained within a confined aquifer or of the water table level of an unconfined aquifer (Figure 1). With a good understanding of the distribution and shape of the potentiometric surface and water table, the subsurface distribution of water can be deduced and estimates obtained of the volume of fresh water contained in an aquifer. Where combined with estimates of hydraulic conductivity (how fast a well recovers after water is evacuated) the potentiometric surface and/or water table level can be used to calculate lateral flow rates within an aquifer. If fully charged, systems I ike that of Guemes Island exhibit potentiometric surfaces and water table levels that rise in the center of the island and fall toward the shorelines. In areas where lateral flow within an aquifer cannot keep pace with water withdrawal, a local depression in the water level is expected.
On Guemes Island, the upper aquifer (Vashon unit) lies above sea level and is unconfined, whereas the more commonly used confined aquifer (Double Bluff unit) lies below sea level. Estimation of the volume of water in the upper aquifer is relatively straightforward if the aquifer thickness and water table level are well known. Calculation of the fresh water volume in the deeper aquifer, however, is more complicated. The depth to the base of an aquifer, where it is thought to rest on underlying impermeable bedrock like that exposed in the hills on the east end of the island, is needed. If the depth to bedrock is great enough (several hundred feet below sea level), then the local elevation of the potentiometric surface is proportional to the thickness of the fresh water lens floating on underlying salt water (Figure A). From this relation, a good estimate of fresh water volume is possible. If the underlying bedrock is shallow, however, a more complex hydrogeologic situation may exist, and the fresh water may not be resting on salt water but rather may be perched on the impermeable bedrock (Figure B). In this case, the overall volume of fresh water may be substantially reduced.
Fortunately, some important data needed to address our questions were obtained during the USGS study. The water level in over 100 wells was determined and recorded as water height measured with respect to the land surface (well head). In addition, several of the wells were monitored over a period of two years to assess seasonal fluctuations in water level, and pump-down tests were conducted on 3 5 wells to assess the hydraulic conductivity of the aquifer.
Nevertheless, we still do not have adequate information to assess the volume and distribution of fresh water within the aquifers beneath Guemes Island. Two critical pieces of information lacking in the USGS study are 1) adequate control of the well head elevation for potentiometric surface and water table level calculation and 2) accurate determination of the depth to bedrock below the island. For the USGS study, elevation control for the measured wells was inferred from topographic maps, which in a best-case scenario can yield a precision of + ten feet. This level of uncertainty is too great for our purposes. Further confounding our efforts, reasonable estimation of the depth to bedrock was not possible at the time of the USGS study because needed geophysical techniques were not available.
A Solution to the Problem
With help from faculty, students and facilities at Rice University, we have the necessary expertise and equipment to supply the missing data-well elevations and depth to bedrock-needed to address our pressing waterproblem.
To establish the elevations of the measured wells and thus determine the potentiometric surface and water table level for the island aquifers, we will employ high-resolution surveying equipment recently acquired with National Science Foundation funds through a grant for my research on deformation of the earth’s crust. The equipment consists of geodetic Global Positioning System (GPS) receivers that, unlike their less sophisticated counterparts used in navigation, can determine horizontal and vertical positions to within a few centimeters.
Regarding measurements of depth to bedrock, we know that the glacial sediments that make up most of Guemes Island and which contain our aquifers are much less dense than the bedrock exposed on the east end. In addition, the magnetic characteristics of the glacial sediments and of the bedrock are significantly different. Using geophysical equipment (gravimeter and magnetometer) from Rice University, we will measure small variations in the gravity and magnetic fields; from this information the depth to the bedrock underlying the glacial deposits can be calculated.
The well elevation data will yield a potentiometric surface and water table level with a precision of a few centimeters, and inversion of the gravity and magnetic data will allow estimation of the depth to bedrock to within several or a few tens of meters. With this data, a good estimate of fresh water volume is possible. When the new data are combined with the hydraulic conductivity determined from the results of pump-down tests, a better understanding of lateral flow within the aquifers should also be possible.
Timing and Scale of the Study
The acquisition of the requisite data should be completed within a year to 18 months, with good first-order models of the island hydrogeology finished in 18 months to two years. The elevation determination of wells will be undertaken during the 1994 Christmas and New Year holiday season. Data reduction for elevation control is relatively simple and will be completed within a few weeks following data collection. The gravity and magnetic survey is a more elaborate process and will be done during the summer of 1995. Data reduction for gravity and magnetics requires the computational facilities at Rice University and should be finished about six months later, by the winter of 1995.
To supplement the study outlined above, I plan to submit a research proposal to the National Science Foundation, Through the proposal I hope to acquire funds to support the project outlined above. (Regardless of the outcome of the funding request, however, the basic aspects of the project will be completed within the outlined schedule.) If research funds are forthcoming, I will expand the project to include more detailed geophysical work and also the drilling of test wells at different sites around the island. More detailed geophysical surveys should yield important insight into lateral variations inthe characterof the glacial deposits, givingus abetter three-dimensional view of the aquifer systems. The test wells would be used to log the subsurface distribution of different rock types within the glacial units and to determine directly the depth to bedrock at the selected sites. The test wells would also serve as important calibration points for hydrogeologic models derived from the USGS data and the newly acquired geophysical data.
Once we have acquired a good estimate of the potentiometric surface and water table level for island aquifers and the depth to impermeable bedrock, models of the fresh water budget can be produced. Reasonable models and predictions shouldbe possible fromthe results ofthe initial study. Additional funding would permit more elaborate modeling. However, even first-order models will yield information that should enhance our ability to make sound judgments as to where new wells are feasible and how much water can be safely withdrawn without adversely affecting our resource.
Implications for Resource Management
Estimation of existing fresh water volume in the two aquifers has obvious implications for responsible resource management. With ourcurrent level ofunderstanding, itisunclearhow freshwater is distributed acrossthe island. Many areas haveno reported problems withwater quantity, particularly those areas with low population densities. In regions of relatively high population density, however, fresh water depletion is evident from pervasive salt water intrusion along shoreline areas.
The assessment of the lateral flow in the two aquifers carries more subtle implications. If we can deter-mine the degree to which different parts of the same aquifer system are connected, we can more realistically appraise the impact that water withdrawal in the interior of the island will have on well systems near the shoreline. If the hydraulic conductivity is high, it is possible that water exploitation in the center of the island could cause rapid depletion of the shoreline portion of the aquifer system, resulting in enhanced salt water intrusion. Alternatively, if hydraulic conductivity is low, interior water withdrawal may have only limited impact on shoreline wells.
The results of this study will allow us to address the water resource of the island as an integrated system rather than as a collection of seemingly independent and unrelated pockets of water. Without adequate data, sound judgments cannot be made. Before drastic measures are taken to solve a possible water shortage on Guemes, the prudent course is first to better understand both the nature of the problem and the physical limitations imparted by the hydrogeology of the island.
– John S. Oldow, Fall 1994