WRRCTR No. 33 Identification of return irrigation water in the subsurface: Water quality.

A joint research effort was undertaken by the Water Resources Research Center and the Honolulu Board of Water Supply to investigate the physical and chemical characteristics of irrigation return water in Pearl Harbor-Waipahu, Oahu, which is an area used for tropical agriculture. The project was started in 1967 and was expanded in 1968 to include Kahuku, Oahu and Central and West Maui. Well samples and profile samples were obtained with a thief sampler in the Pearl Harbor-Waipahu area and composite samples were obtained mainly from pumping wells in other areas. In addition, both spring and stream waters in the Pearl Harbor-Waipahu area were analyzed for a number of inorganic constituents including bicarbonate, calcium, magnesium, phosphate, silica, boron, chloride, nitrate, sodium, potassium, bromide, fluoride, iodine, sulfate, and total hardness. Stream waters in the main study area were observed to contain appreciable quantities of nitrate, phosphate, bromide, bicarbonate, and fluoride. Similar trends were observed with the spring samples including increases of calcium, magnesium, silica, sulfate, and nitrate. Waters from Waiau and Waiawa Springs were found to be closer to sea water in cation composition than that from Kalauao Springs. Well waters from the main study area were evaluated according to Visher and Mink's index constituents, silica, sulfate, and nitrate, and other significant ionic compositions. Well T-241 generally showed higher content of index ions compared to basal water indicating the possibility of its penetrating a caprock aquifer. The waters of T-75 gave evidence of its being in the transition zone between sea water and basal water. T-52 was thought to be in "virgin" basal water. Wells T-202-2C and T-118 are in pumping well fields and show evidence of some freshening due to the draft for municipal use. Well T-191-3A and T-191-3B yielded an erratic pattern that was difficult to analyze, complicated by the fact that the two shafts are about 100 ft apart. General analysis of major constituents evidenced a cyclical trend in concentration, either related to seasonal rainfall and irrigation practices, or both.OWRR Project No. B-012-HI, Grant Agreement No. 14-01-001-1495 The programs and activities described herein were supported in part by funds provided by the United States Department of the Interior as authorized under the Water Resources Act of 1964, Public Law 88-379