Quantifying Sources of Sodium and Chloride

Information on the total amounts of sodium and chloride that entered the Scituate Reservoir drainage basin during WY 2000 were obtained from a variety of sources, including State and municipal public works departments, local precipitation-monitoring programs, State planning programs, and the scientific literature. This information was used to estimate sodium and chloride loads from each major source. The following sections present the methods and assumptions used in each case to develop the load estimates.

Precipitation

Total annual amounts of sodium and chloride that entered the drainage basin in precipitation were estimated from concentrations of sodium and chloride measured in precipitation samples collected weekly at the National Atmospheric Deposition Program/ National Trends Network (NADP/NTN) monitoring site CT15 in Abington, Conn., 15 miles west of the Scituate Reservoir drainage basin, and from monthly measurements of precipitation at five monitoring sites operated by the PWSB (fig. 1). The Thiessen method (Hammer and MacKichan, 1981) was used to weight monthly precipitation totals for each station relative to the area of influence, and these values were summed to obtain the total annual precipitation falling on the drainage basin. This value was then multiplied by the area of the drainage basin to obtain the total volume of precipitation falling on the basin during WY 2000. Precipitation-weighted annual mean sodium and chloride concentrations during WY 2000 at the NADP/NTN site were determined from weekly samples by multiplying sample concentrations by their corresponding sample volumes, summing the values for WY 2000, and dividing by the total sample volume for the year. Total amounts of sodium and chloride were then estimated by multiplying the annual mean concentrations at the NADP/NTN site by the total volume of precipitation that fell on the drainage basin during WY 2000.

Road-Salt Applications

The amounts of sodium and chloride that entered the drainage basin from road deicing were determined from records maintained by the Rhode Island Department of Transportation RIDOT), and by each of the four towns with roads in the basin, of the amounts of road salt applied or purchased during WY 2000. The RIDOT applies a 60:40 mixture of calcium chloride and sodium chloride, together with sand, to State-maintained roads. Before the sand is added, this mixture contains about 16 percent sodium, 62 percent chloride, and 22 percent calcium by weight (Granato and others, 1995). The town of Foster, Rhode Island, used the same reduced-sodium mixture during WY 2000. Amounts of sodium and chloride applied by the RIDOT and the town of Foster during FY 2000 were determined by taking appropriate percentages of the total amounts of salt mixtures purchased during the period. Other towns in the drainage basin used sodium chloride and sand. Records of amounts purchased (Johnston and Glocester) or applied (Scituate), were used to calculate sodium and chloride applications based on the amounts of each constituent in sodium chloride.

Because the total amounts reported by the towns included salting of municipal roads outside of the drainage basin, these values were adjusted by multiplying by the fractions of the total municipal road miles actually present in the drainage basin. Amounts reported by the RIDOT were only for roads within the drainage basin.

Other Deicing Activities

Amounts of sodium and chloride deposited on sidewalks and driveways of private residences were estimated assuming that 50 percent of the households in the drainage basin used an average of 20 pounds of salt (7.9 pounds of sodium and 12.1 pounds chloride) during WY 2000. The number of households (2,254) was estimated by dividing the projected 2000 drainage basin population (see below) by 3.25 persons per average household (Runge and others, 1989). The high utilization rate (50 percent) was chosen to account for additional applications on commercial and industrial parking areas and sidewalks.

Septic-System Leachate

Amounts of sodium and chloride that entered the Scituate Reservoir drainage basin in septic-system leachate during WY 2000 were estimated from the number of people living in the basin and published measurements of per capita ISDS loading rates. Projected 2000 town populations were obtained from the Rhode Island Department of Administration Statewide Planning Program (Rhode Island Department of Administration, 2001) and were used to calculate an estimated percent change in the total population each town. This percent change was applied to the populations in the 1990 census blocks that fell wholly or partially within the drainage basin. These estimates were then adjusted according the fractions of residential-land area RIGIS) in the census blocks that fell within the drainage basin. The total estimated WY 2000 population living within the drainage basin (14,650) was then multiplied by per capita ISDS loads of sodium (21 pounds per year) and chloride (12 pounds per year) reported by Runge and others (1989) for the Scituate Reservoir drainage basin.

Geologic Weathering

Because no measurements of weathering rates for sodium and chloride in the Scituate Reservoir drainage basin were available, estimates were obtained from published values for other sites in New England (Likens and Borman, 1995) and the southeastern United States (Coleman and Diether, 1986). These values range from 0.85 to 1.65 tons per square mile per year. An average reported value of 1.25 tons per square mile per year was applied to the land-surface area of the drainage basin to obtain an estimate of the amount of sodium produced by weathering. It was assumed that the amount of chloride produced by weathering was negligible, because the amounts of chloride in rock types common in the drainage basin are extremely small.