An alternative option involves using groundwater itself as the heat transfer fluid, and drawing this groundwater into the fluid loop for use by the heat pump. This is called an open loop system. These systems are often more efficient than closed loop systems, but do use a substantial amount of groundwater. In Wisconsin, there are regulatory constraints on open loop systems. The main constraint is that the groundwater cannot be reinjected into the earth, but instead must be dumped out onto the surface and allowed to seep back in slowly (to avoid contamination of groundwater supplies). These systems are therefore only possible in some sites.