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Because the total maximum daily load (TMDL) process sets quantitative environmental targets but does not specify the means for achieving those targets, TMDLs are natural candidates for cost-effectiveness analysis. In cases where more than one control strategy can achieve targets, an investigation of cost-effectiveness ratios may indicate the least costly control strategy.

A cost-effectiveness analysis is applied to the N. Bosque River, where impairment is attributed to phosphorus (P) loading from dairy waste application fields (WAFs) and municipal wastewater treatment plants (WWTPs). On average, soluble reactive P (SRP) loads and concentrations need to be reduced by 65 and 70 percent respectively from potential future growth levels, which assume fully permitted flows at WWTPs and fully permitted cow numbers. A baseline scenario and six P control scenarios were simulated assuming fully permitted conditions. Three dairy best management practices (BMPs) were simulated: haul out of dairy manure, field application of manure at the P rate, and reduced P in dairy diet formulations. Phosphorus control at WWTPs and two combined strategies were also considered. The reduced P diet provided benefits (negative costs) to producers, while P control at WWTPs was the next most cost-effective control measure for reducing both P loads and concentrations. These control measures alone, however, do not achieve numeric targets for either SRP loads or concentrations. A combined strategy involving manure application at the P rate on existing WAFs, the haul out of excess manure, and implementation of a reduced P diets in combination with P reduction at WWTPs was estimated to reduce both P loads and concentrations by 87 percent of that needed to achieve targets. In this application, opportunities for reducing the total cost of achieving targets is limited because 1) both P loads and concentrations are targeted, and 2) the levels of those targets require substantial implementation of all control measures considered.