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U.S. GEOLOGICAL SURVEY
Fact Sheet 057-00

IS SEAWATER INTRUSION AFFECTING GROUND WATER ON LOPEZ ISLAND, WASHINGTON?



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HAS SEAWATER INTRUDED INTO LOPEZ ISLAND'S GROUND WATER?

Lopez Island lies among the San Juan Islands, an archipelago in the coastal waters of Washington State, just offshore of Seattle and of Vancouver, British Columbia. Its scenic views and relatively little precipitation have made it one of Washington's premier places to live and play. So its population has been burgeoning, and its interior and shorelines have been under development.

The Island's main freshwater source is ground water. Local surface water cannot be developed to meet increasing needs for freshwater because the Island lacks lakes and continuously flowing streams. But Islanders are concerned that pumping more ground water will affect its availability and quality. Because many wells are near the shores and the recharge rates to the aquifers are low, there is a great potential for seawater intrusion.

In 1997, the U.S. Geological Survey (USGS), in cooperation with the San Juan County Conservation District, studied the possibilities of seawater intrusion on the Island and found that 46 percent of 185 freshwater samples had chloride concentrations indicating seawater intrusion.

THE SOURCE OF LOPEZ ISLAND'S GROUND WATER

Precipitation, mostly rain, is the main source of recharge to the Lopez Island's ground-water system. The Island, shielded by the rain shadow of the Olympic Mountains, receives 20 to 30 inches of precipitation a year, considerably less than other areas of western Washington more directly in the paths of storms from the Pacific Ocean (Oregon Climate Service, Oregon State University, 1999).

Some precipitation is lost to runoff and evapotranspiration. But some precipitation filters downward to recharge the ground-water system of aquifers made up geologically of unconsolidated glacial drift lying over a complex of sedimentary and volcanic bedrock that is metamorphosed in many areas. The glacial drift deposits of sand, gravel, silt, clay, and till cover an estimated 80 percent of the Island and vary in thickness from 0 to as great as 250 feet.

How much of the comparatively little recharge the Island gets each year depends on many factors—the distribution and intensity of the precipitation; the air temperature, and incident solar radiation; the amount and types of vegetation; the slope of the land; the moisture-holding capacity of the soils; and the vertical permeability of the sediments above the aquifers.

But the small amount of yearly precipitation keeps the Island's ground-water system in a fragile balance between the recharge rates and the ground-water pumping. Increased pumping rates may upset this balance and result in seawater intrusion into nearshore aquifers.

Figure 1

Figure 1. Generalized flow pattern of an homogeneous island aquifer. Movement of the zone of transition and water table shown for winter and late summer.

WHAT IS SEAWATER INTRUSION?

In an unconfined aquifer that contacts the sea at the shoreline or seaward, the freshwater, which is less dense than seawater, floats as a lens-shaped layer on top of seawater (fig. 1), and the weight of the overlying freshwater depresses the seawater below sea level. Generally, freshwater recharge in these aquifers moves downgradient and eventually discharges to low-lying coastal areas and into the sea. But pumping out fresh ground water reduces the weight of the overlying freshwater, which in turn can decrease or even reverse the seaward flow so that seawater moves landward into the freshwater aquifer. This migration of seawater into the freshwater aquifer is known as seawater intrusion.

The interface between the salty ground water below and fresh ground water above is a transition zone (fig. 2) of gradually mixing fresh and salt waters. Under natural, undeveloped conditions, the location of this zone will move slightly as the tide rises or falls and as recharge fluctuates seasonally. However, when a well pumps fresh ground water from near the transition zone, the equilibrium can be disturbed and the ground-water flow pattern changed (fig. 2b). As water is pumped out of the water-bearing zone, the transition zone moves upward toward the well. Prolonged or large-scale pumping can raise the transition zone to the well, which may then draw in salty water (fig. 2c).

Withdrawing freshwater from a well affects not only the location of the transition zone around that well but also the location of the Island's regional transition zone. Thus, pumping wells, whether shallow or deep, no matter what their locations, will affect the whole Island's fresh-water system (fig. 1).

The location of the transition zone depends on several natural and human-made conditions: the relative densities of seawater and freshwater; the tides; the pumpage from wells; the rate of ground-water recharge; and the hydraulic characteristics of the aquifer. Because these conditions vary locally, the depth to the transition zone below sea level differs from one place to another on the Island.

WHAT CAN HELP REDUCE SEAWATER INTRUSION?

Seawater intrusion on Lopez Island can be minimized by water conservation, efficient well construction, and by judicious well-operation practices like these:

- Using such water-conserving devices as low-volume plumbing fixtures and toilets.

- Keeping outdoor watering to a minium.

- Reusing or recycling water when possible.

- Augmenting fresh ground-water recharge by, for example, using surface ponds to slow surface runoff and raise infiltration rates.

- Constructing wells that do not penetrate deeper below sea level than necessary.

- Sizing pumps for lower pumping rates and minimizing lengths of pumping cycles.

- In multiple-well systems, pumping wells alternately.

Figure 2

Figure 2. Hypothetical hydrologic conditions before and after seawater intrusion.

WHAT INDICATES SEAWATER INTRUSION?

One indicator of seawater intrusion is an increased chloride concentration in a freshwater aquifer, because chloride, a major constituent of seawater, is chemically stable and moves at about the same rate as intruding seawater. For the purposes of this study, chloride concentrations of 100 milligrams per liter (mg/L) or more were assumed to indicate seawater intrusion.

This study's indication of seawater was also used in a previous USGS study conducted in San Juan County in 1981. That 1981 study used graphical analysis and the cumulative frequency distribution of chloride concentrations to establish a threshold value of 100 mg/L for seawater intrusion (Whiteman and others, 1983).

Seawater contains approximately 35,000 mg/L of dissolved solids, which include about 19,000 mg/L of chloride. Fresh ground water in most coastal areas of Washington generally contains less than 10 mg/L of chloride. Even so, concentrations in excess of 10 mg/L are not conclusive evidence of seawater intrusion because they could be due to airborne sea spray in precipitation, to substantial well pumping rates, to local sources of chlorides, including septic systems or animal manure, or to relict seawater in the aquifer.

At times during the last million and a half years, the sea level along the Washington coastline was higher than now, and the transition zone between fresh and salty ground water was correspondingly farther inland and at higher elevations. Today, occurrences of salt water in Washington coastal aquifers may be due to relict seawater— seawater incompletely flushed from rock materials after the latest decline of sea level. The term relict seawater can also refer to connate water, or water trapped in an aquifer since its formation (Dion and Sumioka, 1984).

1-to-40 and the GHYBEN-HERZBERG PRINCIPLE

In general, if the water table in an aquifer is lowered 1 foot, the freshwater-seawater transition zone will rise about 40 feet, and the total vertical thickness of the freshwater lens will be reduced by about 41 feet (Freeze and Cherry, 1979).

Figure

A century ago, hydrologists working along Europe's coast observed that fresh ground water, appearing to float as a lens-shaped body on seawater, extended below sea level approximately 40 times the height of the freshwater table above sea level. Named the Ghyben-Herzberg Principle after the two scientists who described it, this 1-to-40 relation occurs because freshwater is slightly less dense than seawater (1.000 grams per cubic centimeter (g/cm3) versus 1.025 g/cm3). Thus, for example, if the water table at a given site is 3 feet above sea level, the freshwater-seawater transition zone is 120 feet below sea level, and the vertical thickness of the freshwater body there is 123 feet.

SAMPLING, ANALYSIS, AND QUALITY ASSURANCE

In spring 1997, after USGS scientists had reviewed data from more than 400 possible sites, giving priority to those previously visited (Whiteman and others, 1983 and Dion and Sumioka, 1984), field personnel visited 258 wells and a spring (see table 1) representing the Island.

Water samples from 184 wells and one spring (fig. 3 and table 1) were collected for analysis of specific conductance and chloride concentration. Specific conductance measurements were determined at the USGS Tacoma Field Service Unit (Tacoma FSU), Tacoma, Wash. The chloride content was determined colorimetrically using ferric thiocyanate (Friedman and Erdmann, 1982) at the Tacoma FSU.

Replicate and blank samples were collected and analyzed for chloride at the Tacoma FSU and at the USGS Quality of Water Service Unit in Ocala, Fla., in accordance with the Quality Assurance Plan for Water-Quality Activities of the Pacific Northwest District (Bortleson, U.S. Geological Survey, written commun., 1991). For every six samples, one sample of deionized water blanks was collected and analyzed at the Tacoma FSU. The results for all samples were acceptable. All replicates analyzed at both Tacoma and Ocala agreed within 5 percent of the replicate mean. Reference samples were within 5 percent of the known concentration of chloride. Chloride was not detected in any of the blank samples. The resulting field and quality-assurance data were reviewed and stored in the National Water Information System (Garcia and others, 1997).

WHAT THE STUDY FOUND

The 1997 study found chloride concentrations of 100 mg/L or more in 46 percent of the Island's 185 sites, indicating possible seawater intrusion. Chloride concentrations from the 185 ground-water samples ranged from 12 mg/L to 420 mg/L, with a median value of 92 mg/L.

When the 1997 and the 1981 chloride data were compared, there was no evident change in the areal distribution of chloride values from 1981 to 1997. Both chloride data sets had similar patterns of lowest concentrations near the center of the Island and highest concentrations mainly near the Island's southwestern, western, and northern shores. Moreover, there was no distinct pattern of changes of chloride concentrations in individual wells.

Of wells completed in bedrock units, 56 percent (28 wells) evinced seawater intrusion, while 39 percent of wells completed in glacial drift units (42 sites) showed such signs (fig. 1). One reason the bedrock wells may have shown more seawater intrusion was because they are generally deeper and thus closer to the transition zone.

Table 1. Summary of concentrations of chlorides, physical data, and hydrologic data for wells and a spring sampled in 1997 on Lopez Island[Hydrogeologic unit: B, basalt; G, glacial; M, gravel and basalt; mg/L as CL, milligrams per liter of Chloride; —, no data; . Chloride data rounded to two significant digits, all other data rounded to nearest foot; Altitude of land surface is based on sea level (NGVD of 1927)]

Station name (township/range-section and sequence number) Altitude of land surface (feet) Well depth (feet below land surface) Depth to first opening of well (feet) Spring 1997water-level altitude (feet) Chloride dissolved(mg/L as CL) Hydrogeologic
unit
Spring
1997
Spring
1981
Spring
1978
34N/1W-05H1 40 340 132 41 190 B
34N/1W-05R1 90 400 59 120 G
34N/1W-06B1 100 270 35 120 85 B
34N/1W-06C2 200 200 188 62
34N/1W-06C3 140 315 25 82 B
34N/1W-06L1 40 214 41 10 30 34 28 B
34N/1W-07G2 170 184 184 190 G
34N/1W-07H1 135 174 147 260 170 B
34N/1W-07Q1 40 270 290 150 150
34N/1W-09M1 130 168 163 27 76 G
34N/1W-09P1 105 440 232 270 B
34N/1W-09R1 60 164 30 34 87 87 72 B
34N/1W-16B1 75 143 138 72 150 130 110 B
34N/1W-16D1 150 112 112 72 G
34N/1W-16D3 165 300 110
34N/1W-16G1 90 338 44 66 90 B
34N/1W-17A1 180 198 192 96 G
34N/1W-17B2 180 299 262 19 130 M
34N/1W-17D1 200 250 32 170 100 B
34N/1W-17D2 220 152 18 205 150 G
34N/1W-17E1 70 115 106 13 56 59 52 G
34N/1W-17E2 70 87 82 17 90 G
34N/1W-17G1 150 290 40 200 B
34N/1W-17G2 170 124 19 70 B
34N/1W-17N2 150 142 200
34N/1W-17P1 110 170 99 45 170 100 B
34N/1W-18C1 140 259 102 260 B
34N/1W-18E2 120 260 75 280 350 410 B
34N/1W-18F2 100 35 97 260
34N/1W-18G1D1 80 132 132 6 160 150 G
34N/1W-18H1 100 115 115 82 46 G
34N/1W-18K1 110 15 108 48
34N/1W-18L2 15 46 41 80 G
34N/1W-18L3 20 104 99 260 G
34N/1W-18N1 20 62 62 23 G
34N/1W-18P1 40 58 50 27 280 G
34N/1W-18P2 60 52 47 43 94 G
34N/1W-19N1 70 69 68 48 180 G
34N/1W-19N1S 40 spring 270
34N/1W-20E1 30 164 11 18 52 73 42 G
34N/1W-21E1 80 30 77 110
34N/1W-21H2 70 345 20 38 400 B
34N/1W-21H3 55 64
34N/1W-21M1 80 200 44 260
34N/1W-21M2 100 500 21 250
34N/2W-01M1 260 260 100
34N/2W-02B1 270 266 20 251 42 30 B
34N/2W-02D1 155 307 65 130 140 B
34N/2W-02E1 170 250 160 94 B
34N/2W-02J1 225 300 19 170 B
34N/2W-02J2 205 30 203 12
34N/2W-02P1 140 414 23 230 180 150 B
34N/2W-03A1 150 226 226 70 G
34N/2W-03B1 180 194 194 1 88 G
34N/2W-03C1 200 229 199 86 67 G
34N/2W-03D1 180 216 216 78 G
34N/2W-03F1 130 154 144 150 120 G
34N/2W-03H1 170 204 204 7 86 G
34N/2W-03J1 170 181 176 100 G
34N/2W-03L1 145 265 135 140 92 G
34N/2W-03N1 105 166 166 35 110 B
34N/2W-04B2 82 96 96 92 G
34N/2W-04B4 85 90 90 10 88 G
34N/2W-04G2 80 88 88 11 100 G
34N/2W-04H2 150 169 169 3 100 G
34N/2W-04K1 80 240 52 8 94 B
34N/2W-09A1 62 134 129 210 G
34N/2W-10B1 65 16 94
34N/2W-10C2 90 244 19 92 B
34N/2W-10D1 20 52 46 -4 54 G
34N/2W-10R3 50 330 28 230 M
34N/2W-11A1 125 106 106 87 150 B
34N/2W-11F1 70 272 46 55 110 B
34N/2W-11N4 20 150 39 320 M
34N/2W-12A2 150 40 148 58
34N/2W-12D1 130 134 25 103 130 47 B
34N/2W-12E1 100 179 18 400 B
34N/2W-12G1 150 196 20 132 100 73 B
34N/2W-12M1 110 252 18 280 B
34N/2W-12N1 70 328 20 120 B
34N/2W-12P1 135 265 42 86 180 B
34N/2W-12P3 135 405 33 130 B
34N/2W-13H1 80 12 77 50
34N/2W-13H2 80 180 35 70 170 B
34N/2W-24K1 60 203 20 85 77 M
34N/2W-24L2 60 238 45 160 B
35N/1W-07N1 130 128 118 18 78 G
35N/1W-31D1 75 305 30 60 140 B
35N/1W-31M1 170 245 20 74 B
35N/2W-01M2 60 80 60 15 360 G
35N/2W-01N3 40 64 58 8 110 G
35N/2W-01P3 40 50 11 300
35N/2W-02P1 40 60 55 100 50 G
35N/2W-02P2 70 99 94 -8 180 G
35N/2W-02P3 70 76 71 5 100 G
35N/2W-10B3 40 33 28 19 30 G
35N/2W-10G1 120 135 130 -2 120 G
35N/2W-10J2 170 180 180 7 100 G
35N/2W-10K1 140 135 125 19 100 G
35N/2W-10Q4 150 153 153 17 130 G
35N/2W-10Q5 160 150 151 150 G
35N/2W-11A1 60 104 98 12 110 G
35N/2W-11B1 60 130 -60 54
35N/2W-11C2 90 102 97 8 30 34 G
35N/2W-11D1 60 68 63 32 39 34 G
35N/2W-11F1 130 143 138 34 G
35N/2W-11J1 170 185 185 4 34 45 G
35N/2W-11K1 145 160 160 0 80 G
35N/2W-11N1 130 154 38
35N/2W-12B3 60 74 74 8 48 G
35N/2W-12C3 130 141 136 32 G
35N/2W-12D2 90 95 90 8 30 44 G
35N/2W-12D3 130 158 152 28 G
35N/2W-12E2 220 243 243 12 32 G
35N/2W-12F2 22 29 3 140 90
35N/2W-12L2 60 70 70 4 80 57 56 G
35N/2W-12M2 165 186 186 -2 80 G
35N/2W-12P1 18 40 34 1 110 G
35N/2W-12Q1 60 80 80 6 200 G
35N/2W-12Q2 60 66 66 18 100 G
35N/2W-12R1 165 181 180 -3 190 G
35N/2W-13B1 100 116 116 2 74 G
35N/2W-13D1 115 125 125 6 78 G
35N/2W-13E1 100 114 115 3 40 G
35N/2W-13M1 100 114 114 3 48 G
35N/2W-13R1 85 151 146 4 160 G
35N/2W-14A2 130 150 150 4 48 G
35N/2W-14B2 135 164 164 -0 62 G
35N/2W-14E1 100 113 108 -0 50 39 27 G
35N/2W-14F1 135 142 136 10 32 G
35N/2W-14J2 100 160 100 -2 100 G
35N/2W-14M1 110 113 105 10 52 G
35N/2W-14M2 180 155 136 45 50 G
35N/2W-14N1 170 137 132 48 120 G
35N/2W-15B1 130 158 153 290 200 G
35N/2W-15H1 70 102 102 -2 90 G
35N/2W-15R3 20 65 61 0 18 G
35N/2W-21J1 20 302 23 250 B
35N/2W-22D1 15 150 10 86 150 B
35N/2W-22J1 45 50 46 16 63 73 B
35N/2W-22L1 18 345 20 -9 94 B
35N/2W-23C1 80 100 99 0 32 G
35N/2W-23D1 80 99 89 46 G
35N/2W-23G1 160 173 173 3 14 B
35N/2W-23G2 182 216 216 -12 30 B
35N/2W-23J2 220 229 211 12 30 G
35N/2W-23K2 205 211 211 5 30 G
35N/2W-24A2 98 265 106 33 150 G
35N/2W-24E1 100 120 112 110 G
35N/2W-24H1 120 100 110 130 92
35N/2W-24K1 80 90 85 35 100 G
35N/2W-24P1 250 302 302 67 100 G
35N/2W-24Q1 44 91 86 26 100 G
35N/2W-25B1 40 141 95 32 39 74 G
35N/2W-25F1 139 237 83 40 B
35N/2W-25P1 190 300 40 50 60 B
35N/2W-25Q1 125 60 60 84 82 64 G
35N/2W-25R1 60 215 26 49 94 87 B
35N/2W-25R2 60 382 318 220 B
35N/2W-26B1 218 220 220 9 64 G
35N/2W-26D1 290 322 296 10 40 G
35N/2W-26G1 265 280 275 10 20 G
35N/2W-26K1 230 261 261 1 40 38 G
35N/2W-26M1 220 249 241 30 25 G
35N/2W-26P1 190 426 426 -7 160 G
35N/2W-27E1 33 38 38 5 30 G
35N/2W-27E2 22 22 18 13 12 G
35N/2W-27F2 70 78 78 12 40 G
35N/2W-27F3 40 9 30
35N/2W-27J1 230 241 241 102 28 G
35N/2W-27J2 250 270 165 -5 20 G
35N/2W-28K3 110 122 117 8 150 G
35N/2W-28Q1 190 212 207 -3 60 72 G
35N/2W-28R1 150 155 155 11 60 55 G
35N/2W-33G1 130 140 140 15 420 360 390 G
35N/2W-33J2 190 220 -6 150
35N/2W-33R2 190 190 189 16 24 G
35N/2W-34F1 180 194 188 -1 68 75 G
35N/2W-34K1 190 203 203 4 100 G
35N/2W-35D1 205 191 191 22 28 G
35N/2W-35H1 280 255 30 270 28 38 B
35N/2W-35L1 210 310 131 28 23 B
35N/2W-35M1 185 219 219 3 100 G
35N/2W-36D1 290 485 300 50 51 M
36N/2W-36N1 80 149 20 24 B

Figure 3

Figure 3. Areal distribution of chloride concentrations from wells or spring on Lopez Island measured in the spring of 1997.


Table 2. Chloride concentrations in ground-water samples collected April 1981 and in late April to early June 1997,Lopez Island, Washington[Chloride data rounded to two significant digits]

Group of samples Year Number of samples Chloride concentration, in milligrams per liter P-value
Minimum 25th
percentile
Median 75th
percentile
Maximum
1981 44 23 45 67 100 360
Paired, all 10.01
1997 44 16 42 86 140 420
Paired, 1981 28 34 54 86 140 360
within 1,500
feet of
shoreline
10.06
1997 28 16 59 90 160 420

1 A Wilcoxon signed-rank test (one-sided) was used to test the hypothesis that the chloride concen-trations in 1997 were not greater than chloride concentrations in 1981. The test was conducted using onlywells that were sampled in 1981 and again in 1997. P-values less than 0.05 indicate a significant increase in chloride concentrations from 1981 to 1997.

When seawater intrudes, three trends are usually apparent. First, chloride concentrations at a given site may increase over time. Second, for wells open at the same depth, there may be a strong relation between chloride concentrations and a well's distance from the shoreline, with chlorides being greater the closer a well is to shore. Third, chloride concentrations at a given site may increase with depth.

The first trend was found on Lopez Island. Chloride samples collected from the same wells in 1981 and in 1997 showed a statistically significant increase in concentration over time (table 2). But no trends were found between chloride concentrations and distance from shore or between chloride concentrations and the depth of a well's open interval. These trends may not have been apparent because of wells too shallow to be strongly influenced by the freshwater-saltwater transition zone or because of the effects of sea spray, varying lithologies, different ground-water levels, possible pumping before sampling, or uneven areal distribution of sampled sites.

The 1981 and 1997 chloride data were subjected to two statistical tests: Wilcoxon signed-rank tests on (1) all paired samples and (2) paired samples from near-shoreline wells within 1,500 feet of the shoreline (Helsel and Hirsch, 1992). Using paired samples removes the influence of many environmental factors, so the test more accurately indicates real differences in chloride concentrations over time. The paired samples from near-shoreline wells were tested because one may expect the wells closer to the shoreline to be more sensitive to seawater intrusion. The wells tested for chlorides in 1981 and 1997 showed a statistically significant increase in concentration. But no significant increase in concentration was found for the near-shoreline wells (table 2).

Chloride concentrations in excess of 100 mg/L suggested seawater intrusion, and the statistical tests indicated that concentrations had increased over time. But the data did not show trends of consistently higher concentrations near the shoreline or consistent increases of concentration with depth. Thus, further investigations are needed to rule out sources of chloride other than seawater intrusion.

HOW CHLORIDE AFFECTS THE QUALITY OF THE WATER?

According to the U.S. Environmental Protection Agency (EPA), water with high chloride content may, among other things, cause high blood pressure; taste salty; corrode pipes, fixtures, and appliances; and blacken and pit stainless steel. The EPA has set a Secondary Maximum Contaminant Level (SMCL) of 250 mg/L for chlorides. An SMCL is the concentration limit for a nuisance contaminant that could affect the aesthetic quality of water by causing taste, odor, or staining problems (U.S. Environmental Protection Agency, 1996).

FUTURE STUDIES

Future studies like these examples could help assist understanding of seawater intrusion on Lopez Island:

CITED REFERENCES

Dion, N.P., and Sumioka, S.S., 1984, Seawater intrusion into coastal aquifers in Washington, 1978: U.S. Geological Survey Water-Supply Bulletin 56, 10 of 14 pls.

Freeze, R.A., and Cherry, J.A., 1979, Groundwater: Englewood Cliffs, N.J., Prentice-Hall Inc., 604 p.

Friedman, L.C., and Erdmann, D.E., 1982, Quality assurance practices for the chemical and biological analyses of water and fluvial sediments: U.S. Geological Survey Techniques of Water-Resources Investigations, Book 5, ch. A6, 181 p.

Garcia, K.T., Maddy, D.V., Lopp, L.E., Jackson, L.D., Couope, R.H., and Schertz, Terey L., 1997, User's manual for the National Water Information System of the U.S. Geological Survey, chap. 2: U.S. Geological Survey Open-File Report 97-634, unpaginated.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in water resources: New York, Elsevier Science Publishing Company, 522 p.

Jones, M.A., 1985, Occurrence of ground water and potential for seawater intrusion, Island County, Washington: U.S. Geological Survey Water-Resources Investigations Report 85-4046, 6 pls.

Oregon Climate Service, Oregon State University, 1999, 1961-1990 annual average precipitation contours: Washington, accessed June 2, 1999, URL http://www.ocs.orst.edu/pub/maps/Precipitation/Total/States/WA/wa.gif.

U.S. Environmental Protection Agency, 1996, Drinking water regulation and health advisories: U.S. Environmental Protection Agency, Office of Water, EPA 822-R-96-001, about 12 p.

Whiteman, K.J., Molenaar, Dee, Bortleson, G.C., and Jacoby, J.M., 1983, Occurrence, quality, and use of ground water in Orcas, San Juan, Lopez and Shaw Islands, San Juan County, Washington: U.S. Geological Survey Water-Resources Investigations Report 83-4019, 1-12 pls.

ACKNOWLEDGMENTS

The USGS thanks the many well owners and well drillers who supplied well records and other information and allowed access to their wells, and to Dave Garland of Washington State Department of Ecology.

Adapted & edited by James Lyles, designed by Connie Dean, illustrated by Deanna Walth, and formatted by Ginger Renslow.

FOR MORE INFORMATION CONTACT

USGS: Science for a changing world - Logo

Laura Orr, U.S. Geological Survey
1201 Pacific Avenue, Suite 600, Tacoma, Washington 98402
(253) 428-3600 http://wa.water.usgs.gov/

San Juan County: Health and Community Services - Logo

San Juan County: Conservation District - Logo

San Juan County Conservation District
350 Court Street #10, Friday Harbor, Washington 98250
(360) 378-6621 FAX: (360) 378-2445

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