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<br />Because a greater amount of salt is being suppl ied to the irrigated <br />land than the Rio Grande is removing, an unfavorable dissolved- <br />solids or salt balance exists above Alamosa. This undesirable <br />characteristic is detriment91 to the irrigated land, and proper <br />management of the areas affected is necessary in order to maintain <br />the proper plant growth and crop yield. Below Alamosa the salt <br />balance is more favorable, as the river removes more salt from the <br />surrounding area than it contributes. <br /> <br />The qual ity of water in the Rio Grande near Lobatos is the result <br />of all factors affecting the river as it flows through the basin. <br />The observed minimum and maximum dissolved-solids concentration and <br />percent sodium, October 1946 to October 1965, are shown in Table D-2. <br /> <br />Figure D-2 shows the 1946-65 average monthly discharge and the <br />average monthly discharge-weighted dissolved-sol ids concentration <br />and percent sodium for water in the Rio .Grande near Lobatos. <br />Generally, the discharge and the dissolved-solids concentration <br />is high. The somewhat anomalous peaks in dissolved solids and <br />percent sodium in March are probably the result of overland runoff <br />from snowmelt within the basin. This runoff dissolves the "alkal i" <br />(mainly sodium sulfate) from the soil as the water drains into the <br />Rio Grande. Overland runoff probably accounts for similar peaks in <br />August. <br /> <br />2. Ground Water <br /> <br />The chemical quality of water in the unconfined aquifer is the <br />result of many complex and interrelated factors. These factors <br />include: chemical quality of the recharge water, mineralogy of the <br />soils and sediments through which the water moves, and the concen- <br />trating effect of evapotranspiration. <br /> <br />The chemical quality of water in the unconfined aquifer is excellent <br />around the rim of the basin. The specific conductance of the water <br />is usually less than 250 umhos/cm, and the water is a calcium <br />bicarbonate type. As the ground water flows toward the center of <br />the basin, a deterioration in water quality occurs. Continued <br />dissolution of soluble minerals from the soils and sediments as the <br />water is recirculated through the irrigation cycle causes the <br />dissolved-sol ids concentration to increase. in areas where the <br />depth to water is less than 12 feet (3.7 m), evapotranspiration <br />removes virtually distilled water from the unconfined aquifer <br />causing further increase of dissolved solids in the water remaining <br />in the aquifer. <br /> <br />Nitrate was detected in excessive concentrations in the unconfined <br />ground water of the Rio Grande fan. The high concentration of <br /> <br />f'- <br />,-,'", <br /> <br />~, <br />.~Q <br /> <br />D-4 <br />