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OO,05~ B. Water Quality Impacts There are two types of water quality impacts that result from etliciency changes: changes in the assimilative capacity of the stream and changes in the pollutant load entering the stream. Irrigation return flows pick up sediment, dissolved minerals, and agricultural chemicals as they travel across the field and through the soil. Return flows then add pollutants to the watercourse. The size of the pollutant load depends on farming methods, soil types, underlying geology, and distance from the stream, but in general any reduction in return flows will result in a lower pollutant load entering the stream. The assimilative capacity of the stream measures its ability to absorb a given pollutant load without adversely impacting water quality. This process is more complex than simple dilution, because it depends on more than just the volume of stream flow (i.e., temperature, biological activity, chemical composition, etc.). However, in general terms, the ability of a stream to assimilate wastes does improve when the volume of stream flow increases. This tension between water quality and water quantity is already recognized in a number of ways, and may eventually limit diversion and use of surface water in Colorado. The precise effect of irrigation efficiency changes on the assimilative capacity of a stream will depend on downstream uses, types of pollution discharges entering lower reaches from other sources, . timing of other discharges (i.e., seasonal irrigation return flows, intermittent industrial discharges, or year round municipal waste discharges), and the quantity of flows in the stream. For instance, the loss of return flows may reduce late fall and Winter flows in smaller streams, making tbem less able to assimilate the discharges from municipal users, but if no one else uses the stream during this period, the water quality reduction may be tolerated. 27