This was originally posted to: Guemes Island Environmental Trust

Water, Water Everywhere

Part Two

ABSTRACT: Twelve water samples, ten from wells and two from surface accumulations, were analyzed to determine the isotopic composition of oxygen in a preliminary study of the ground water underlying Guemes Island. The results of this study indicate that the ground water is derived from rainwater infiltration and thus is a renewable resource. Unfortunately we do not know the rates of recharge versus withdrawal for the ground water, and, given the pressure for growth and development on the island, we run the risk of depletion our water supply.

Variations in the isotopic values of the ground water occur and define broad geographic domains over western and central Guemes. The apparent lack of mixing of ground water between different domains suggests the existence of several aquifer systems that lack island-wide connections. Considering the large but imprecisely known amount of surface area of the island that does not allow water infiltration (areas of standing water and/or no “perc “potential), the possibility of non communicating aquifer systems beneath the island poses a substantial resource management problem. Several ground water recharge areas probably exist, but their locations currently are unknown. Definition and preservation of these areas is critical in the maintenance of a source of fresh water for island residents. A complete hydrological study of Guemes Island is needed before additional growth can be accommodated responsibly.

Introduction

As promised in the last issue to the newsletter, the results of the oxygen-isotope study of water samples from wells and surface sources on Guemes Island are presented here.

To ascertain whether or not our fresh water supply was derived, at least in part, from old water trapped during the last ice age.

The study focused on two fundamental questions concerning the resource potential of the sole source aquifer system underlying our island. The first was to ascertain whether or not our fresh water supply was derived, at least in part, from old water trapped during the last ice age, The second question was the degree to which aquifers are in communication and thus, able to transfer water from one part of the island to another.

To begin with, it is important to recognize that the results presented here are preliminary and that the accompanying interpretations are derived from limited data. This study only is an initial step in formulating a better understanding of the water resource of the island. We must develop a comprehensive understanding of the water resource potential of Guemes and its capacity to supply the needs of the residents.

This article is divided into two parts. The first part, the Overview, addresses the results of the work without significant attention given to the underlying science and technology. The Technical Information section in the second part is for readers who wish to gain more insight into the method used and the reasoning behind the interpretations.

Overview

Parameters of the Study

In December of 1989, ten water samples were collected from wells located in the central and western parts of the island (Figure 1); the area that is underlain by fluvial (river and stream environments) and glacial deposits of sand, gravel, and clay. No samples were taken from the eastern end of the island where wells are drilled in bedrock. During the sampling, an effort was made to distribute the collection sites over a wide area to better assess the spatial variability in the composition of the ground water. The water samples were taken from well sources that had not been filtered or purified, thus allowing a better reading of the natural condition of the water in the underlying aquifer. Two samples of surface water were collected to calibrate the composition of runoff water.

 

The bulk chemistry of the water samples was not analyzed in this study, but previous work by Dave Garland of the Washington State Department of Ecology indicates that the ground water of those parts of the island underlain by poorly consolidated sedimentary deposits is dominated by sodium and chloride with subordinate amounts of carbonate, magnesium, and calcium. The objective of the study presented here was to determine the isotopic composition of oxygen in the water. The analyses were conducted this spring in the stable isotope laboratory at the Department of Geology and Geophysics at Rice University where I am a professor. For each of the samples, the isotopic data, the depth of the well (where known), and the altitude of the wellhead above sea level (derived from United States Geological Survey quadrangle maps) are presented in Table 1.

Interpretation and Implications

First the good news: the oxygen isotope data for the ground water are consistent with values expected for rainfall for Puget Sound. Thus, a safe conclusion is that the ground water beneath Guemes is meteoric (recharged from rainfall). We all should feel relieved because the fresh water of our island is a renewable resource.
Unfortunately, what we still don’t know is the rate of water recharge versus the rate of withdrawal. As most people on the west and north shores have known for years, the withdrawal, at least during the summer season, surpasses the recharge rate. This results in salt water intrusion. If over the years the consumption of water greatly increases in these areas, it is possible that the winter rains will not be sufficient to recharge the aquifer adequately. Although salt water intrusion is not yet a serious problem on the south shore, continued development may soon overtax the fresh water source resulting in similar salt water intrusion problems. Ultimately, there may be no fresh water source to accommodate existing homes or future growth for the entire island.

Now for some bad news. A depth versus isotopic ratio plot presented in Figure 2 and the distribution of sample sites in Figure 1, yield some interesting, if not completely clear results. As any professional scientist is prone to say in this situation, more data are needed before formulation of conclusive statements. However, certain propositions can be made.

The isotopic data appear to define two major populations (A and B) with the single shallow well near the center of the island suggesting a third (C). The geographic distribution of the isotopic values for the subsurface sample sites is consistent with an interpretation that the water for populations A and B are derived from two noncommunicating aquifer systems. Considering the overlap in the range in depths (Figure 2) recorded for the wells for each population, however, the water within the aquifer systems apparently is not simply isolated at different depths by intervening layers of impermeable clay. Rather, it seems that the aquifers for a given area are connected vertically and lack island wide lateral connections.

It is clear that we have a complex hydrologic system on Guemes Island. Island residents must become aware of this complexity and, more importantly, recognize the incredible degree of ignorance that exists concerning one of our most fundamental resources. Before significant future development occurs on Guemes, we must better understand our sole source aquifer system. We must fully assess the recharge rates in order to determine the population density that a given aquifer system can sustain. To estimate the water infiltration into the aquifers, we must know the location of the various recharge and runoff areas.

Once the hydrology of Guemes is understood, we will face another dilemma. With the identification of several recharge areas, how do we propose to protect them? They undoubtedly lie in private property. Possibly the island will have to purchase the parcels and define the areas as water resource sites. The first step, however, is to initiate a hydrology study.


Technical Information

In this section, the isotopic data are discussed and placed in the framework of a geologic model that presents a plausible geometry for the aquifer systems underlying Guemes. At the onset, however, a little background on isotopic systems is needed in order to understand the significance of the oxygen isotope data in water resource assessment.

Oxygen Isotopes of Water

Most elements (e.g., 0, oxygen; S, sulphur; U, uranium; Pb, lead; etc.) have forms that vary in atomic weight but which have the same atomic number and thus have essentially the same chemical characteristics. The isotopes of various elements come in three varieties depending on their origin and stability. Two types of isotopes, radioactive and radiogenic, are more familiar to most people as they represent the source and/or product of fission and fusion reactions. An additional class of isotope exists and neither is the product of radioactive decay nor has the property of spontaneous decay. These are stable isotopes. For oxygen, three stable isotopes exist but only two are quantitatively important. The most abundant isotope of oxygen is 16O, with an atomic weight of 16, followed by the heavier 18O which is only 0.002 times as abundant as 110. The third isotope, 17O, is found in very minute amounts and is unimportant for our purposes.

The chemical characteristics of an element are determined by the atomic number, so that the different isotopes (different atomic weights) behave virtually the same during most chemical reactions. The difference in atomic weight, however, can contribute to minor deviations in the physical properties that result solely from the variation in mass. These differences in physical properties are significant only among the isotopes of the lighter elements where the variations in the atomic weight are proportionally large with respect to the mass of the atom. The differences in mass cause isotopic fractionation which is any process that causes the isotopic ratios to differ due to chemical or physical processes.

For natural water, the ratio of the two main isotopes, 18O/16O, averages about 1:500 (or 0.002). In most isotopic studies, it is not convenient to use absolute rations and what is called the delta notation is used instead. Delta18O is the difference between the isotopic ratio of 18O/16O of a given sample and the 18O/16O of a standard divided by the ratio of the standard all multiplied by 1000. This relation is presented in the equation below:



The standard is known as SMOW (standard mean ocean water), and if a given sample has the isotopic signature of SMOW the value of delta 18O is zero. If the sample has a different isotopic ration than SMOW, the delta 18O value will vary between positive and negative.

Oxygen Isotopes and Guemes Island

The ratio of the primary oxygen isotopes for water varies in response to numerous chemical and physical processes. Of importance to this study of ground Water on Guemes, however, is the natural isotopic fractionation of the oxygen isotopes during evaporation and precipitation. As outlined in the first section of this article, one of the primary goals of this study was to ascertain whether or not the water in our sole source aquifer was charged by recent rainfall or if some component was fossil water left over from the last ice age.

This question is addressed by comparing the isotopic ratio of the ground water derived from the well water samples with that of surface water and known isotopic rations for rain water in the Puget Sound region. Globally, rain water has different delta 18O values which are dependent primarily on atmospheric circulation patterns and latitude. In equatorial regions, where evaporation is rapid, natural isotopic fractionation occurs with a slight depletion of 18O with respect of 16O in the vapor phase. Thus, the moisture forming clouds has a delta 18O that is negative. During condensation, reverse fractionation occurs and the 18O is preferentially removed as part of the liquid as rain, and the first rain to fall from these clouds has an isotopic value like that of SMOW. As rain continues to form, however, the 18O is selectively removed, resulting in a progressively more negative delta 18O in the rain water.

In the Puget Sound region today, the values of delta 18O in rainwater range between - 10 and - 13. During the last ice age, however, the delta 18O for Puget Sound was on the order of -15 to -20. With such a difference between the isotopic ratios, it is relatively simple to determine whether or not the ground water underlying Guemes Island is meteoric (recharged from rainfall) or if it is partially derived from old water.

Use of the isotopic data is also important in determining the degree of communication among aquifers. If all of the ground water of the island had the same isotopic ratio, it would be reasonable to propose that the aquifers are connected and that the water is mixing. On the other hand, if the isotopic signature for specific parts of the island have different values, it is possible that several aquifer systems exist, and that they are not in communication with one another.

Discussion of the Isotopic Data

The data from the ten well samples and two surface samples are presented in Table I and on Figure I which illustrates the geographic distribution of the sampling sites. The delta 18O data is plotted against the depth of the source aquifer (assumed to be the bottom of the well) and is shown on the graph in Figure 2. Three populations (A, B, and C) appear and have values of about - 12.5 to - 12.9 for the first (A), - 11. 3 to - 12.0 for the second (B), and about -10 for one sample (C). Two subsurface samples are from wells of unknown depth, but their delta 18O values, plotted as dashed lines in Figure 2, lie within the two dominant populations (A and B). The surface water samples (plotted as circled dots) lie within the expected range of values for Puget Sound.

It is clear from the graph in Figure 2, that the bottoms of all but one of the wells sampled lie between 20 and 100 feet below sea level. When the subsurface isotopic data are viewed on the map in Figure 1, the isotopic populations define geographic domains. For the shallow-source well near the center of the island (Figure 1) the isotopic data yields the least negative value (about -10). The northern and east-central parts of the island are supplied by relatively deep wells (-20 to about -60 feet) which contain water of the isotopic population A. An apparent enclave exists along the south shore of Guemes where even deeper wells (-40 to -90 feet) have water of population B. Given the overlap between the depths of wells containing isotopic ratios lying within populations A and B, it appears that the isolation of the two populations is not a simple matter of depth variation of the supplying aquifer. Also note that for the two wells with unknown depths, their 81110 values and locations are consistent with the geographic distribution of populations A and B (domains A and B in Figure 1).

Origin of the Isotopic Variation

The variation in isotopic composition of surface water samples lies well within the expected values for Puget Sound and western Washington and thus poses no problem. Explanation of the spatially segregated populations observed in the subsurface samples, however, requires additional discussion. The differences in the measured values of delta 18O are well within the analytical uncertainty of the laboratory at Rice University and thus, represent a real variation in isotopic composition. The fact that the different values lie within the expected and observed range for surface water for the region could suggest that the differences are not statistically meaningful, however. This would be a reasonable explanation if it were not for the spatial segregation of samples with similar compositions. The data indicate that within the two deep ground water domains internal mixing occurs but that limited or no mixing takes place between the two fresh water reservoirs.

A possible source of the isotopic variation of the ground water within the geographic domains could be chemical interaction with the material of the host aquifer. Much of the well water on the island is acidic as indicated by the common occurrence of hydrogen sulfide (rotten egg smell) and would readily react with any calcium carbonate within an aquifer. Reaction with calcium carbonate would affect the isotopic composition of the oxygen in the water. Considering the small difference in observed values, however, only a small amount of calcium carbonate could be involved. Alternatively, the less negative values observed in the wells along the south shore may indicate a greater interaction of fresh and salt water in this area. Additional geochemistry is needed if these questions are to be answered. Nevertheless, it is clear that at least two and possibly three or more aquifer systems exist on Guemes and that they are not undergoing significant mixing.

Guemes Island Aquifer System

The isotopic data appear to indicate the existence of laterally discontinuous aquifers beneath Guemes Island. Surprisingly, the isotopic and well-depth data suggest that, within specific geographic domains, ground water is mixing over substantial vertical distances. This suggests laterally isolated yet vertically communicating aquifer systems.

It is clear that at least two and possibly three or more aquifer systems exist on Guemes and that they are not undergoing significant mixing.

The scenario of vertically connected aquifer systems is possible geologically, even if untested on Guemes Island. Justification for this possibility can be found in the cliffs along several of the island beaches. In most situations, relatively young and undeformed sedimentary deposits, like those underlying the central and western parts of Guemes, can be assumed to be composed of a vertical succession of horizontal layers that are continuous laterally over substantial distances. In this case, sand and gravel layers, acting as aquifers, are separated by relatively impermeable clay layers. This results in a vertical stratification of the aquifers. If this simple case held for Guemes, however, we would expect all wells tapping an aquifer at a given elevation with respect to sea level to have similar isotopic ratios for oxygen. This is not observed and necessitates explanation.

The exposures of strata along several of the higher relief shore lines around the island illustrate the possible source of vertically communicating aquifers. In cross sectional view, like that found on sea cliffs, the layers of sand, gravel, and clay (and occasional coaly layers; lignite) can be traced for substantial distances horizontally. It is important to note, however, that in many cases a specific layer is discontinuous laterally. This can be due either to its truncation by higher (younger) layers that fill channels that eroded into the underlying strata during deposition (Figure 3) or due to a primary lenticular character of the layer (e.g. sand or gravel channels). Thus, the lateral extent of layers (sand and gravel of the aquifers and intervening clay of the aquitards) is discontinuous horizontally.

The next step in this visualization is to expand the two-dimensional view to three dimensions. The lateral extent of an individual layer can be interrupted in a direction into a cliff as well as along the cliff face. The discontinuity of layers in three dimensions can yield a complex geometry in which several aquifers can be in contact with one another. It is conceivable that individual aquifers, which may be laterally continuous over a large, but spatially isolated, area can be connected vertically through a complex system of down-cutting stratigraphic layers. Intervening clay layers that restrict the flow of ground water may also merge in the vertical dimension, thus effectively forming impermeable walls that could serve to isolate aquifer systems.

These relations are show schematically in Figure 3. In this diagram various stratigraphic layers, composed of sand, gravel, and clay, are depicted in a block diagram. Where clay resides near the surface, rain water is indicated as runoff. A recharge area, illustrated as being underlain by a sand layer, captures part of the runoff and rainfall by water infiltration through the topsoil. Hypothetical paths of water migration in the subsurface are shown by arrows and indicate how water could pass down through the various porous strata to the deepest aquifer. The deepest aquifer is illustrated as a lenticular body of sand (possibly deposited in a stream or river environment) that is surrounded by relatively impermeable layers of clay except where the overlying gravel deposit filled an old erosional channel. Note that the younger gravel channel, which cuts down through the underlying clay layer, represents the only conduit for ground water recharge in the lower aquifer. The diagram illustrates in a simplistic way a means in which vertical communication can exist amongst aquifers that have restricted lateral extent.

Assessment of this geologic model requires detailed study of the log records of the wells on the island. With such an analysis, coupled with additional geochemical study, it may be possible to better define the boundaries of the various aquifer systems underlying Guemes Island. Linking this information with a careful study of the surface geology can establish the location of the recharge areas for each ground water system. This information combined with data on withdrawal and recharge rates for specific aquifer systems can be used to develop an accurate resource assessment and plan for water management for the island.

-John Oldow

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