In this paper we build on a voluntary mechanism introduced by Segerson and Wu (2006) that uses the threat of an ambient tax to induce nonpoint source polluters to reduce emissions. In addition to reviewing Segerson and Wu's policy, we propose a new voluntary/threat policy that offers some theoretical advantages over that put forward by Segerson and Wu. We then test various forms of voluntary/threat policies in the experimental economics laboratory and compare the outcomes to a policy that administers strictly a per unit ambient tax.