Nonpoint sources in the lower Missouri River

Dynamic variations in the chemical end isotopic character of the lower Miss ouri River can be explained primarily in terms of the relative contribution of lower basin waters in the river, which can be computed from gauging sta tion data to vary from < 5 to 86%. The endmember representing the lower bas in component has an average electrical conductivity of 261 <mu>S, a pH of 7 .8, an alkalinity of 111 mg/L sulfate (SO4) of < 5 mg/L total organic carbo n (TOC) of 5.9 mg/L, and a <delta>O-18 value of -6.1, mostly similar to sha llow meteoric groundwaters in Missouri. The endmember representing the uppe r basin has an average electrical conductivity of 814 muS, a pH of 8.2, an alkalinity of 191 mg/L SO4 of 216 mg/L, TOC of 3.0 mg/L, and a delta O-18 v alue of -11.6. The normalized volumetric contributions facilitate geographi c delineation of nonpoint source pollutants such as TOC and offer new ways to predict water quality end estimate annual loads.