We consider a model of regulation for nonpoint source water pollution through non-linear taxation/subsidization of agricultural production. Farmers are heterogenous along two dimensions, their ability to transform inputs into final production and the available area they possess. Asymmetric information and participation of farmers to the regulation scheme put constraints on the optimal policy that we characterize. We show that a positive relationship between size of land and ability may exacerbate adverse selection effects. We calibrate the model using data on a French watershed and we simulate the optimal second-best policy and characterize the allocation of the abatement effort among the producers.