Page images
PDF
EPUB

Cost-Effectiveness of the National Stream-Gaging Program

The U.S. Geological Survey began a nationwide analysis of its stream-gaging program during fiscal year 1983. The purpose of the analysis is to define and document the most cost-effective method of furnishing streamflow information. The analysis is being carried out over a 5-year period with about 20 percent of the program being analyzed each year. The Survey operates about 8,000 continuous-recording gaging stations nationwide that provide streamflow information for a large variety of users. These gaging stations will be evaluated (1) to identify the principal uses of the data and to relate these uses to funding sources, (2) to identify alternate, less costly methods of furnishing needed information, and (3) to define strategies for operating the program to minimize the standard error in streamflow data while staying within the operating budget. The first two steps are designed to ensure that sufficient need exists for operating a gaging station. The third step provides for allocation of financial and manpower resources among the stations that remain in the program after the screening process, so that the program is operated in the most cost-effective manner. An analysis completed for the State of Maine early in fiscal year 1983 will serve as a prototype.

In the first step, the known uses of streamflow data generated at a gaging station are compared against the objectives of the stream-gaging program to ensure sufficient justification exists for Survey involvement at that station. Deficiencies in the existing data-collection program are evaluated to ensure that all information needs are met. The responsiveness of the operation of each station to the types of uses also is evaluated to see that streamflow information is timely. For example, analysis of the data uses for 51 stations in Maine indicated that three stations should be discontinued as soon as is practical and that an additional three stations should be discontinued at the end of short-term projects. Analysis also indicated that, as funds become available, additional stations should be established in the interior of Maine to better define regional hydrology.

The second step of the analysis is used to determine whether sufficient streamflow information can be generated at a station

by methods other than operating a continuous-record station. Primarily, two alternate methods are considered, a flowrouting model and a statistical regression model. The flow-routing model uses the traveltime of flow between stations, the storage in the stream channel, and hydrologic routing techniques to transfer daily flows from an upstream station to a downstream one. The statistical regression model correlates daily flows at the station of interest with daily flows at other nearby stations. Once calibrated, both models can be used to estimate daily flows at discontinued stations by using daily flows from operating stations. The accuracy of the estimated streamflow must be suitable for the intended usage for an alternate method to be viable. In the Maine analysis, there was one station where both models provided daily discharges of sufficient accuracy for the intended usage. Both models were calibrated using all existing data, and the recommendation was to continue operating the station until sufficient data were available to verify the models.

The final step is used to determine the best allocation of money and manpower among the stations that remain in the program after the two screening steps. Because there are so many uses made of streamflow data, minimization of the standard error of streamflow data, expressed as percentages, is chosen as the general measure of the program's effectiveness. This part of the analysis defines the uncertainty function for each station in the program, develops the necesary cost information, and determines the number of visits necessary to each station to minimize the uncertainty.

The uncertainty function relates the standard error of streamflow data to the number of visits and measurements made per year or season. Examples of typical uncertainty functions from the Maine study are given in figure 1. These uncertainty functions are computed using a statistical technique that evaluates the accuracy of the streamflow rating curve, the accuracy of transferring flows from nearby stations, and the variability of historical flows at the station. The rating curve at each station is the relationship that enables the hydrologist to convert the

140 TTTTTTTTTTTTTTTTT

Figure 1. Typical uncertainty

functions for three gaging stations in Maine.

[graphic]

STANDARD ERROR, IN PERCENT

[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]

20

10

15 NUMBER OF VISITS AND MEASUREMENTS recorded water-surface elevation (gage standard error relationship with the budget height or stage) to streamflow. At some can be developed. Figure 2 is an example sites, additional correlative data are

of an uncertainty-cost relationship for necessary to determine the flow, such as Maine. The original Maine stream-gaging the fall in water surface between sites.

program, consisting of 51 stations, operWhen the recorder at the station fails to

ated with an annual budget of $ 211,000. record the water-surface elevation (or

As a result of the data-use analysis, it was other correlative data), the rating curve

recommended that 6 of the original 51 cannot be used, and daily flows must be stations in the Maine stream-gaging proestimated from flows at nearby sites or

gram be discontinued. The stream-gaging from historical flows at the station. The program analyzed for cost-effectiveness uncertainty function includes the variability consisted of 45 stations. The current or standard error of flows estimated in

criteria for operating the 45-station prothese various ways.

gram require a budget of $180,300. This Once the uncertainty functions for each

is the circle in figure 2 marked "Current station are known, various costs associ

Practice." The average standard error of ated with stream gaging can be deter

the streamflow records was 17.7 percent. mined. Feasible routes are defined for serv

As can be seen in figure 2, this overall icing the stations, and each station is

level of accuracy could be maintained with assigned to one or more routes. The cost

a budget of about $ 170,000 if allocation of servicing each station, route costs, and

of resources among the gages was altered. the minimum number of times each station

The recommendation was to modify the should be visited are determined. The fixed operation of the program and to use the costs of operating each station, including

residual $10,300 to increase receipt of the cost of computing records and their

data from the interior of the State. The storage and publication, are also deter

relationship in figure 2 indicates the reducmined. This information and the uncertain

tion in uncertainty that can be achieved by ty functions are input to a computer pro increasing the total budget. gram that determines the number of times Studies like the one in Maine are schedeach route is used. The routes selected are uled for completion in 17 States during those with the largest reduction in uncer fiscal year 1983. The entire stream-gaging tainty per dollar of expenditure. By varying program will be analyzed over the next 5 the total budget and repeatedly running years as part of the continuing effort of the program, an uncertainty or average the Geological Survey to evaluate the Na

[merged small][graphic][subsumed][merged small][merged small][merged small][merged small][merged small]

Contamination of Ground Water by Coal-Tar
Derivatives in St. Louis Park, Minnesota

Operation of a coal-tar distillation and surface and is overlain by glacial drift, two wood-preserving facility in St. Louis Park, bedrock confining beds (Glenwood and Minnesota, from 1918 to 1972 resulted in basal St. Peter), and two bedrock aquifers severe ground-water contamination. In

(Platteville and St. Peter). Nonetheless, it is 1978, the U.S. Geological Survey began now contaminated because materials detailed studies of the transport and fate entered the aquifer through at least five of coal-tar derivatives through ground

wells that hydraulically connect more than water in the area. Local, State, and Federal one aquifer (multiaquifer wells). The single agencies will use the results of the studies major source is a well on the former plant to guide management decisions and to site that was drilled in 1917 to an original design remedial action. The studies were depth of 909 feet. When first geophysicalconducted in cooperation with the Min

ly logged by the Survey in 1978, the well nesota Department of Health, Minnesota had filled to a depth of 595 feet. The upPollution Control Agency, city of St. Louis permost 100 feet of the fill was mostly Park, and the U.S. Environmental Protec coal tar. Moreover, approximately 150 tion Agency.

gallons per minute of contaminated water The problem of most immediate concern was moving through the well bore from to the city and to the State and Federal the St. Peter aquifer into the Prairie du regulatory agencies is the presence of tox Chien-Jordan aquifer. ic organic compounds in water withdrawn

Contaminants in the Prairie du Chienfrom some municipal wells. When the first Jordan aquifer have moved at least 2 miles municipal well was drilled in 1932, the

northeast and southeast of the plant site. Prairie du Chien-Jordan aquifer contained The direction and rate of contaminant water having a distinct coal-tar taste. The movement changes with time because the well is 3,500 feet from the plant site.

bedrock ground-water flow system continFrom 1978 to 1981, use of seven more ually adjusts to hydraulic stresses caused municipal wells in this aquifer was discon by water withdrawals and flow through tinued because the wells yielded water

multiaquifer wells. Contaminants move containing trace amounts of coal-tar com rapidly through the Prairie du Chienpounds, including at times and places, the Jordan aquifer because the upper part is a carcinogen benzo(a)pyrene.

carbonate rock with fractures and solution The Prairie du Chien-Jordan aquifer is channels. Consequently, the concentration the region's major ground-water resource. and composition of contaminants in water About 75 percent of ground-water with pumped from the Prairie du Chien-Jordan drawals in the St. Louis Park and Minne aquifer through individual industrial and apolis-St. Paul metropolitan areas are

municipal wells fluctuates with time. from this aquifer. The aquifer has good

Contaminants entered the uppermost natural protection from near-surface

bedrock aquifer, the Platteville, directly sources of contamination. In the St. Louis from the drift and moved at least 4,000 Park area, it is 250 to 500 feet below land feet from the plant site. Locally, the con

Degradation of phenolic
compounds by bacteria

Movement of
Retardation of movement

contaminants
Vertical movement
of polynuclear aromatic

into underlying
of coal-tar fluids
hydrocarbons by sorption bedrock aquifers

Downgradient
Flow of uncontaminated
Dissolution of

through bedrock

migration of
ground water
coal tar by

valleys and

biorefractory ground water

multiaquifer cells

compounds

[merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small]
[graphic]

taminants have reached the St. Peter aquifer through the Glenwood confining bed and (or) through bedrock valleys where the confining bed has been removed by erosion.

The greatest mass of contaminants is in the drift near the plant site. Coal-tar derivatives reached the water table by percolation through the unsaturated zone and through ponds that received surface runoff and process water from the plant. Parts of the drift contain an undissolved liquid mixture of many individual coal-tar compounds. Chemical analyses of organic fluid and water from a monitoring well completed in the drift 50 feet below the water table identified more than 200 individual organic substances. The viscous organic fluid is denser than water and has moved slowly downward independent of the

Vertical aerial photographs of

the plant site during plant operation (left) and redevelopment (right).

« PreviousContinue »