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I I I I I I I I I I I I I I I I I I I ~ t-. '0 :0 WA TE R QUAL! TV The quality of surface and ground water resources in the Region reflects both natural and manmade influences. Problems include high mineral contents, low dissolved oxygen levels in surface waters due to municipal wastewater discharges, high turbidities and sediment loadings, salinity encroachment in fresh water aquifers, excessive nutrient concentrations, and elevated bacterial levels. Surface Water Water quality standards for temperature, chemical and biological consti- tuents, and certain physical properties of water have been established for all surface water bodies in the Rio Grande Region [23,24,25]. These standards are continually being reviewed and updated in accordance with changing con- ditions and problems. Those presently in effect in the three states are listed in Table 5 for specific stream segments; new standards for Colorado streams in the Rio Grande Basin are presently being proposed. The segments in Table 5 were designated by the states for water quality planning and pollution control purposes. As part of the nationwide water quality improvement program administered and partially funded by the Environmental Protection Agency (EPA), stream segments are classified according to whether or not adopted water quality standards are violated. Segments are classified "effluent limited" when water quality is sufficient to meet applicable standards or when there is adequate demonstration that applicable standards will be met after required federal effluent limitations are satisfied. All other segments are classified "water quality limited", and they represent those where appli- cable standards are being violated and will continue to be violated after implementation of federally-mandated pollution controls. The present classifications of stream segments in New Mexico and Texas in the study area are identified in Table 6 [26,27,2B,29,30,31]. Similar classifications for Colorado stream segments were not available. As noted in the table, the majority of the segments are classified effluent limited, indicating acceptable water quality conditions. Several of the mainstem segments of the Rio Grande, the Rio Chama and the Pecos River 35