Historically, policies for controlling water pollution have been concerned with conventional pollutants discharged into surface waters. Three primary types of water are susceptible to contamination: surface water, Groundwater, and oceans. Policy purposes are useful to distinguish between two sources of contamination— point and nonpoint— even though the distinction is not always crystal clear. Three primary sources of ocean pollution are oil spills, ocean dumping, and trash that end up in the ocean. Airborne pollutants, such as sulfur dioxide, mercury, and nitrogen, eventually find their way to rivers and lakes via atmospheric deposition. Fund pollutants are those for which the environment has some assimilative capacity. The most troublesome cases of pollution result from stock pollutants, which merely accumulate in the environment. The Water Pollution Control Act of 1948 represented the first attempt by the federal government to exercise some direct influence over what previously had been a state and local function.