| Groundwater
Quality
Agriculture and
the Lower Nooksack River Basin1
Nitrate contamination of ground water from land use activities,
especially agricultural activities, is common in many parts
of the lower Nooksack River Basin. Fifty-five of the 368 wells
sampled for nitrate showed concentrations above the U.S. Environmental
Protection Agency and Health Canada drinking water standards.
A USGS report (Cox and Kahle 1998)2
describes a study conducted in cooperation with the Whatcom
County Planning Department covering about 225 square miles of
northern Whatcom County and southeastern British Columbia. Data
from over 600 wells were used to assess the regional ground
water system of that area and evaluate sources of nitrate found
in its ground water.
Agricultural activities, which occur over about 75 percent
of the land area, were estimated to contribute from 85 to 88
percent of the total nitrate loading to ground water. Loading
from residential and domestic sources contributes an estimated
6 to 7 percent.
Potential sources of nitrate include dairy and poultry farming;
fertilizers applied to croplands, lawns, and gardens; irrigation
water; and natural sources such as rainfall, peat, soil, and
leguminous plants. On a per-acre basis, the rate of nitrate
entering the ground water from agricultural sources is roughly
1.5 to 3 times greater than residential sources.
Elevated concentrations of iron and manganese also were found
in numerous wells, but their presence is due to natural interactions
of ground water with the rock and organic matter in the aquifer.
Six pesticide or volatile organic compounds were detected in
4 of 24 wells sampled for organic compounds as part of this
study. In three wells, one compound was detected at concentrations
below drinking water standards. In the fourth well, four compounds
were detected, including ethylene dibromide (EDB) and 1,2-Dichloropropane
(1,2-DCP), which were detected at concentrations above drinking
water standards. This well is located in an area of previously
reported pesticide detections by the Washington State Department
of Ecology.
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Soil Fumigant-Derived
Compounds: Whatcom County3
In another study, the Puget Sound Basin National Water Quality
Assessment (NAWQA) project is conducting a land-use study to
assess the effects of raspberry production on ground water in
some of these aquifers, and a flow-path study to estimate denitrification
rates in one aquifer. A summary of the project follows below.
The glacial outwash aquifers in the northern portions of Whatcom
County and Canada typically have shallow depths to the water
table and are very permeable. These factors coupled with the
intensive agricultural activity in this region have made the
aquifers vulnerable to contamination.
Previous studies of these aquifers show that nitrate concentrations
in ground water often exceeded the maximum contaminant level
(MCL) for drinking water. Ethylene dibromide (EDB) and 1,2-dichloropropane
(1,2-DCP) were frequently detected in samples, and concentrations
of both compounds exceeded MCLs in certain areas (Erickson and
Norton 1990; unpublished data collected by the Washington State
Department of Health and the USGS).4
The presence of 1,2-DCP and EDB in ground water is related to
the past usage of soil fumigants on crops. In two separate actions
(1983 and 1984) EPA canceled all major agricultural uses of
EDB. Soil fumigants currently used in northern Whatcom County
include metam sodium (sodium N-methyldithiocarbamate), Telone
II (1,3-dichloropropene with trace amounts of 1,2-DCP), and
methyl bromide.
The Puget Sound Basin NAWQA land-use and flow-path wells were
sampled during 1997. 1,2-DCP was found in water from 22 of 35
wells sampled, and although concentrations were generally low,
concentrations in two samples were above the MCL. Concentrations
of nitrate in water from about 34 percent of sampled wells exceeded
the MCL. EDB and methyl bromide were not detected. The continued
presence of 1,2-DCP in ground water prompted the Whatcom County
Executive to send a letter to Cynthia Barton, Washington District
Chief, requesting a study to help delineate the extent of ground-water
quality problems in the area. In contrast to the NAWQA land-use
study, this study would evaluate the effects of fumigant applications
to all crops, not just raspberries. The sampling would be coordinated
with sampling planned by state and local agencies. This proposal
represents work that will help address the needs of Whatcom
County, and at the same time augment the ongoing NAWQA land-use
and flow-path studies.
Objectives
The objectives of this study are to 1) provide additional information
on the presence and concentrations of nitrate and soil-fumigant-related
compounds in ground water in the Fishtrap and Bertrand Creek
watersheds, and 2) attempt to correlate past usage of soil fumigants
with the presence of the fumigants or their breakdown products
in ground water. It is hoped that the study will lead to recommendations
regarding the use of soil fumigants over sensitive aquifers
such as those in northern Whatcom County.
Relevance and Benefits
The project was coordinated with the Puget Sound Basin NAWQA
study of fumigants in ground water in an agricultural area in
Whatcom County and British Columbia, Canada. Two elements of
the project, collecting additional groundwater samples for analysis
of fumigants and collecting fumigant application data, made
a more comprehensive assessment of the effects of fumigants
on ground-water quality possible.
Approach
Objective 1: A total of 18 ground-water samples collected from
existing wells in the Fishtrap and Bertrand Creek watersheds
will be analyzed for nitrate, fumigants, and fumigant breakdown
products. Included are EDB, 1,2-DCP, methyl bromide, breakdown
products of 1,2-DCP (chloropropene, 1,2,2-trichloropropane,
2-chloropropane, and 1-chloropropane), and breakdown products
of metam sodium (methyl isothiocyanate and methyl sulfide).
Field measurements will be made to determine water temperature,
specific conductance, pH, and the concentration of dissolved
oxygen. Two field blanks will be submitted for analysis.
Objective 2: The presence or absence of fumigants and their
breakdown products in ground water sampled from the 18 wells
and from about 30 NAWQA land-use and flow-path wells will be
related to fumigant applications overlying and upgradient of
sampled wells. Application data for the years 1991-1997 will
be obtained from the Washington State Department of Agriculture.
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Seawater Intrusion:
Lopez Island5
Chloride concentrations in the island's ground water indicate
that seawater intrusion has likely occurred in some wells, according
to a report published by the U.S. Geological Survey, Department
of the Interior (USGS).
Lopez Island, one of the San Juan Islands, does not have the
lakes and perennial streams to supply more fresh surface water,
and it gets only 20 to 30 inches of precipitation a year to
recharge its ground water. But the island's growing population
and continued development are increasing demands for fresh water.
"Pumping more fresh water from wells would at some point
lower the water table. And that could raise the transition zone
between the fresh water and seawater enough to bring seawater
into the freshwater aquifer," said the USGS's Laura Orr,
author of the report.
Because of such concern, the USGS, in cooperation with the
San Juan Conservation District, launched a 1997 study of the
potential of seawater intrusion on Lopez Island. In 1997 the
USGS collected samples from 185 selected well or spring sites
on the island and analyzed the samples for chloride concentrations.
Of the 185 samples, 46 percent had chloride concentrations
in excess of 100 milligrams per liter, indicating possible seawater
intrusion. The concentrations ranged from 12 mg/L to 420 mg/L,
with a median of 92 mg/L. Moreover, the concentrations had increased
over time since a previous USGS study in 1981.
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References Cited
1. U.S. Geological Survey (USGS). Agriculture
Indicated as Largest Source of Nitrates of the Lower Nooksack
River Basin's Ground Water, New USGS Report Says. <http://www.usgs.gov/public/press/public_affairs/press_releases/pr1057m.html>
2. Cox, E., and S. C. Kahle. 1999. "Hydrogeology, groundwater
quality, and sources of nitrate in lowland glacial aquifers
of Whatcom County, Washington and British Columbia, Canada."
U.S. Geological Survey Water Resources Investigations Report
98-4195.
3. U.S. Geological Survey (USGS). Study of Fumigant-Derived
Compounds in Shallow Ground Water, Whatcom Co., WA . <http://wa.water.usgs.gov/wadmin/Projects/summary.417.htm>
4. Erickson, D., and D. Norton. 1990. "Washington State
Agricultural Chemicals Pilot Study, Final Report." Washington
State Department of Ecology. Publication No. 90-46.
5. U.S. Geological Survey (USGS). News Release: May 8, 2000.
Seawater may have intruded
into Lopez Island ground water, says USGS report. <http://wa.water.usgs.gov/reports/news/news.fs057-00.html>
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