occur and what happens to the daughter chemicals produced is an important part of investigations sponsored by the Toxic Substances Hydrology program.

Changes induced by sunlight in the chemical form of iron, for example, were measured in a stream receiving acid mine drainage near the headwaters of the Arkansas River in Colorado. These photochemical reactions are responsible for daily changes in the chemical form of iron that have been overlooked in previous studies of acidic streams and lakes. These overlooked changes may account for the reports of wide variations in concentrations of iron, copper, and other metals in acidic streams and lakes.

In South Dakota, daily fluctuations in pH caused by photosynthetic reactions affect the concentrations of dissolved arsenic in Whitewood Creek. This reaction is significant because dissolved arsenic is much more readily taken up by organisms than is arsenic associated with particulate material.

Biologically induced transformations are studied extensively at many of the field sites. Investigations of a crude-oil spill site in Bemidji, Minn., show an anaerobic (oxygenfree) degradation of oil components, a process previously thought to be unimportant. At a Pensacola, Fla., creosote site, natural microbiological reactions appear to be removing contaminants. Study of biological degradation at this site will show the potential of using biological remediation of contaminated ground water at other sites.

Many important environmental processes occur at the interfaces between air and water or between solids and water. The transfer of chemicals across an interface can have a profound impact on the transformation and transport rates of these chemicals and on their availability to organisms.

For example, at the Whitewood Creek field site, arsenic accumulates in benthic invertebrates, and, in the Calcasieu River study in Louisiana, synthetic organic compounds accumulate in clams, fish, crabs, and aquatic plants. Careful analysis of contaminant concentrations in water, sediment, and tissue taken

from the Calcasieu River tracks the mechanisms of contaminant uptake. This information is useful in predicting the effects of chemicals on organisms living in contaminated environments.

Interface reactions can also be important in removing contaminants from the environment. The loss of organic chemicals via volatilization (vapor) from unconfined groundwater contamination sites is demonstrated for chlorinated solvents at Picatinny Arsenal, N.J., and for crude oil components at the Bemidji site. In Minnesota, volatilization is the most important loss mechanism for the crude-oil components and their degradation products.

Knowing how quickly and in what concentrations chemicals move away from the site of contamination is important to environmental managers. In the case of ground water, contaminant movement may be only a few feet per year; whereas, in surface water, movement may be tens or hundreds of miles per year. In the Whitewood Creek and Arkansas River study areas, variable flow conditions affect the transport of trace metals and arsenic in streams. The transport and accumulation of trace metals in surface-water sediments over time can be determined by examining historical concentration profiles in reservoirs that receive contaminants from upstream sources.

At the Cape Cod, Mass., site, relating the movement of colloidal-sized bacteria to water movement has shown the importance of transport mechanisms of chemicals that are not diluted or broken down in ground water. This is important because many organic chemicals of environmental concern may travel as or with colloidal material.

The hydraulic connection between surface and ground water can be an important route for contaminant transport. Contaminated surface water, such as waste-disposal pits or evaporation ponds, can be a source of pollutants to ground water. The reverse is also true. At the Globe, Ariz., site, ground water contaminated by copper mining is discharging into a perennial stream. Scientists have successfully predicted the timing of the

appearance in the stream of some of the individual chemicals in the contaminant plume. This knowledge is not only valuable to local managers, it can also be used to improve general computer models of contaminant transport. The models can in turn be used at other sites that have similar conditions and problems.

Studies under the Toxic Substances Hydrology program outline the environmental processes that control the transformation, availability, and transport of surface- and ground-water contaminants. Although much of the work has been conducted at a small number of field sites, the accumulated knowledge of the processes controlling contaminant behavior can be applied on a wider scale at other contaminated sites. The program continues to provide information that is useful in mitigating existing and future ground- and surface-water contamination problems throughout the country and in various geographic settings.

Atrazine in Streams of the Midwestern

United States

By Donald A. Goolsby

A

trazine is one of the most extensively used pre-emergent herbicides in the United States. About 80 million pounds of atrazine are used each year, about 70 percent of which are applied in 12 Midwestern States for weed control in the production of corn and sorghum. Atrazine and its metabolic degradation products are being transported into surface- and ground-water resources of this region.

A study in Iowa, conducted by USGS scientists in cooperation with the Iowa Department of Natural Resources, shows that 18 percent of 355 municipal wells contain detectable concentrations of atrazine. In municipal,

domestic, and irrigation wells in Minnesota, Missouri, and Nebraska, atrazine is detected in 30 percent or more of the wells sampled. Atrazine concentrations in many of the wells exceed 3 micrograms per liter, the proposed maximum contaminant level (PMCL) for drinking water established by the U.S. Environmental Protection Agency.

Until recently, most concern about contamination of water resources with atrazine and other agricultural chemicals has been focused on ground water; contaminated surface water has received scant attention, except in a few small watersheds. To develop information on the occurrence and distribution of

atrazine and other agricultural chemicals in surface water in the Midwest, USGS scientists conducted a reconnaissance in 1989 under the Toxic Substances Hydrology program (see p. 53).

2

About 150 streams in 122 hydrologic cataloging units, geographically distributed across 10 Midwestern States, were sampled three times-before herbicide application, during runoff just after herbicide application, and in the fall during low-flow conditions. The median size of the drainage basins sampled is about 800 square miles and the aggregate drainage area of the basins is about 200,000 square miles. All of the samples were screened for atrazine, and most were subsequently analyzed for 10 additional herbicides and 2 atrazine metabolites (see table).

Results from the reconnaissance show that detectable concentrations of atrazine persist year round in most streams throughout the Midwest. During spring and early summer runoff following herbicide application, atrazine concentrations increase by one to two orders of magnitude and then decrease to pre-application levels by fall. For an undetermined time following herbicide application, more than one-half of the streams sampled had atrazine concentrations higher than the 3 micrograms-per-liter PMCL, and more than one-fourth of the streams had concentrations of 14 micrograms per liter or higher. Because of the random design of the reconnaissance, these results are believed to be typical of streams throughout the Midwest.

Low concentrations of atrazine were detected in 76 percent of the stream samples collected in the fall when streamflow is derived primarily from ground water. This result strongly suggests that the alluvial

100

10

Pre-application

о

Post-application

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Fall low-flow

A

0

0.5

1.5

Pre-application

aquifers contributing baseflow to these

streams are contaminated with low concentrations of atrazine and are a nonpoint source for contamination of streams during baseflow periods. This result also underscores the need for further study of the exchange of water and contaminants between surface- and ground-water systems.

Atrazine was the most frequently detected of the 11 herbicides and 2 metabolites measured during each of the three sampling periods (see table). The second most frequently detected compound was the atrazine metabolite deethylatrazine. This compound is not applied as an herbicide but is largely derived from the breakdown of atrazine by soil microorganisms. Notably, little information is available at this time about the toxicity and health effects of deethylatrazine or other atrazine metabolites. The ratio of deethylatrazine to the parent compound, atrazine, is lowest in late spring and early summer after herbicide application and highest in fall when the principal source of water in streams is from ground water. This ratio may prove to be useful in determining sources and mechanisms for atrazine contamination of surface and ground water.

USGS hydrologists are continuing research on the occurrence, distribution, and fate of atrazine and other agricultural chemicals in water resources of the Midwest. Current research includes studies of the temporal distribution of several herbicides, including atrazine and its metabolites, in spring and summer storm runoff; the occurrence, distribution, and deposition rates of atrazine in atmospheric wet deposition; and the regional distribution of atrazine and other agricultural chemicals in ground water.

Drought in California, 1987-90

By Richard A. Hunrichs

alifornia marked its fourth consecutive year of drought in 1990. Precipitation, runoff, and reservoir storage have been below normal during each water year from 1987 to 1990. Stream runoff in the Sacramento River basin in the relatively wet northern part of the State is a major source of water for State and Federal water-supply projects. Runoff is also a widely used indicator of the status of the State's water supply. On the basis of runoff in the Sacramento River basin, California Department of Water

Resources (DWR) personnel classified the 1987, 1988, and 1990 water years as critically dry.

A critically dry year is that which occurs about 1 year in 10 in the record of a particular region. Runoff during the 1989 water year was below average but was sufficient to prevent the year from being classified as critically dry. As late as March (the last month of California's winter storm season), watersupply forecasts showed a greater than 50 percent chance that 1989 would be a critically dry year. A series of March storms, however, brought plentiful rain and snow over the northern half of the State and temporarily moderated the severity of the drought.

Historic Perspective

Droughts are a recurring feature of California's climate. To help put the current drought in perspective, hydrologists from the USGS water resources office in Sacramento, in cooperation with DWR personnel, examined streamflow records from the Sacramento River basin and from unregulated long-term stations located throughout California. When considered individually, no single year of the current drought ranks as extreme. When considered as a whole, however, the drought aspect of the last 4 years is more evident.

For the Sacramento River basin, two consecutive critically dry years have occurred only twice before, from 1933 to 1934 and from 1976 to 1977. Three consecutive critically dry years are unprecedented in the historical record, and three critically dry years during a 4-year period have occurred only once before, from 1931 to 1934, during the Dust Bowl Era. The current 4-year drought has three critically dry years, two of which are consecutive (1987-88, 1990). Even though the drought is less severe in southern California, that area's dependence on transfers of water from the northern part of the State gives the current drought a statewide importance.

Prior to the drought of 1987–90, the droughts of 1928-37 and 1976-77 were considered to be the most severe in the State's history. Comparing the current drought to these historic droughts provides perspective, but comparisons are difficult because the droughts have different durations. The severity of multiyear droughts can be measured by the accumulated deficit in streamflow (departure below the mean) during the drought. Periods of drought of the same duration can be directly compared by ranking their accumulated deficits.

Ranked by accumulated deficits, the droughts of 1928-37, 1976-77, and 1987-90 (in the central part of the State) are equivalent in severity. The drought of 1928–37 is

PERCENT OF LONG-TERM AVERAGE

160

140

120

ZZ Runoff

Legend

Precipitation

End of Year Reservoir Storage

100

80

60

40

20

0

Summary of water conditions in California, 1986-90 water years. Values are in percent of average. (Data from the California Department of Water Resources.)

WATER YEAR

the State's most severe drought, however, when the duration, the magnitude and areal extent of accumulated deficits, and the severity of individual years are taken into account. The current drought will have to continue for several more years before it would rival the drought of 1928-37.

Impact of the Drought

Because of the recurrent nature of droughts in California, considerable effort has been made to lessen their effects. Water supplies have been developed and are managed to provide dependable supplies to major agricultural areas and population centers and, more recently, for the protection of environmental quality. Isolated dry years and the first year of protracted droughts have only limited adverse effects on human activities. The most seriously affected areas are wildlands and nonirrigated agricultural lands. Water-supply problems were minimal in 1987, however, because surface-water storage was carried over from 1986.

During 1988 and early 1989, water shortages affected about one-third of California's population and more than 40 percent of the State's irrigated agriculture. Many areas had insufficient rainfall for dry-farmed crops, and ranchers from 42 counties were accepted into Federal emergency feed programs. Drought emergencies were declared in 14 counties. Many urban areas instituted mandatory or voluntary water conservation measures.

Where available, ground water was used to compensate for deficiencies in surface-water supplies. In general, ground-water supplies were adequate, but water shortages occurred in localized areas of excessive drawdown and in some upland and coastal areas where ground-water reservoirs are small.

In 1989, managers in both of the State's two major water projects, the State Water Project and the Federal Central Valley Project, announced anticipated reductions in water deliveries of as much as 50 percent. The wet weather in March 1989 brought relief to many water users, however, and permitted full delivery of agricultural water supplies. Drought restrictions were eventually lifted in most areas of the State. Water restrictions continued along the central coast, which did not benefit from the March rains and is

not a part of any of the State's large water projects.

In 1990, no relief from spring rains was forthcoming. Deliveries of State and Federal project water are reduced as much as 50 percent for agricultural customers, and some municipal and industrial contractors have lesser reductions. Only once before (1977) in the history of the water projects were such reductions necessary.

In response to the reductions in surface water, farmers are modifying irrigation practices, turning to ground-water supplies (often an expensive alternative), taking land out of production, and changing crop rotation. Reduction in irrigation water causes salt to