This paper estimates the cost of a policy to restrict water trades to mining firms in northern Chile to protect riparian ecosystems and indigenous agriculture. In response to the policy, mining firms have developed high-cost desalination and pumping facilities to secure adequate water supplies. We develop a methodology and estimate the cost of market transactions that fail to occur due to the policy. Lost trade surplus is estimated at $52 million per year. Without trade restrictions, around 86% of the remaining agricultural water in the region would be transferred to mining.