Concentrations of nitrate, chloride, and total solids in water in the Magothy aquifer, southeast Nassau County, N.Y., show a steadily increasing trend from the early 1950 's through 1973. Vertical distribution of nitrate, chloride, and total-solids concentrations as shown in cross sections of the study area indicate downward movement of these constituents from tens of feet to a few hundred feet in this time period. Maximum concentrations are in a zone underlying the areas of Westbury, Hicksville, and Plainview. Nitrate (as nitrogen) concentration increased from 4-5 mg/liter to 7 mg/liter in the area of Westbury and from 3 to 10 mg/liter in Plainview during the period 1950-73. Lowest nitrate concentrations are in the area south of a line running from North Merrick to South Farmingdale. Also, during the period analyzed (1950-73), a 10-mg/liter line of equal-chloride concentration on a cross section in the Westbury area moved downward a distance of less than 50 feet, and in the area of Hicksville, nearly 150 feet. Total-solids concentration doubled during the period 1950-73 in the area of Plainview, where maximum downward movement of pollutants was observed. (Woodard-USGS)