The water in the Delaware (and the streams and tributaries that feed it) comes from precipitation.

The continuous movement of water between the earth and the atmosphere is known as the hydrological cycle. Under several influences, of which heat is predominant, water is evaporated from both water and land surfaces and is transpired from living cells. This vapor circulates through the atmosphere and is precipitated in the form of rain or snow.

On striking the surface of the earth, water follows two paths (in amounts determined by the intensity of the rain and the porosity, permeability, thickness, and previous moisture content of the soil). The first path, termed surface runoff, flows directly into rivers and streams and thence into oceans or landlocked bodies of water. The second path is infiltratation into the soil. A part of the infiltrated water becomes soil moisture, which may be evaporated directly or may move upward through the roots of vegetation to be transpired from leaves. The portion of the water that overcomes the forces of cohesion and adhesion in the soil profile percolates downward, accumulating to form the groundwater reservoir, the surface of which is known as the water table. Under natural conditions, the water table rises intermittently in response to replenishment, and then declines as a result of continuous drainage into natural outlets such as springs. The other form of drainage is the wells used to extract ground water for human consumption.

Using Pennsylvania as an example: The precipitation in the state ranges from about 32 to 60 inches annually, but if these extremes are averaged the entire state receives about 41 inches on each of its 45,309 square miles. In an average year the precipitation totals about 33,000 billion gallons. In a drought year the precipitation available falls to about 22,600 billion gallons, and in a wet year the amount may exceed 40,000 billion gallons. In an average year, this amount of water provides about 728 billion gallons of water for each square mile of land. In a drought year, the amount falls to about 500 billion gallons, and in a wet year it will exceed 880 billion gallons a square mile. Of the water from precipitation, about only half is returned immediately to the atmosphere by evaporation or transpiration, about one-third infiltrates the soil and bedrock, and less than one-fifth runs off the land into streams and rivers.
The availability of ground-water is directly related to the geology of the area. The type of rock - sedimentary, metamorphic, or igneous - and the geologic structure are the controlling factors. The porosity of the different classes of rocks is important in groundwater yield. Igneous and most metamorphic rocks are extremely dense and are usually poor aquifers. Springs from these rocks are rare, and wells drilled into them do not yield large quantities of water. Of the sedimentary rocks, sandstones are normally major sources of groundwater. Some wells in sandstone may be more than 1,000 feet deep. Limestones and shales, because of their low porosity, are usually poor aquifers. The unconsolidated sand and gravels of the Atlantic Coastal Plain are good aquifers. (1.)

Below are a few of the many sources of pollution. More will be listed as more of the exsisting research has been read.
Hazardous Wastes: In the past, many industrial plants disposed of hazardous waste materials by dumping their waste materials on the ground, burying the wastes in containers that did not provide permanent storage, or dumping the wastes in nearby streams. Many of these waste sites are now known to have materials containing carcinogens. The removal of hazardous sites has been amazingly slow. As the problems of removal linger on, there are important ramifications for land use and health.
Raw Sewage: While it has long been recognized that raw sewage presents a health problem, development of sewage treatment systems still lags in many areas. In addition, many of the older sewage treatment plants are inadequate and must be improved to meet established health standards. This process of modernization will take many years. When improper treatment of waste materials occurs in an area, it often results in the need for water to be boiled for human consumption or water being obtained from nearby communities.
Household refuse: Generated from normal household functions, the refuse is disposed of either by burning or by controlled disposal in landfills. The latter requires a detailed knowledge of the geological nature of the subsoil and the proximity of groundwater. The potential for hazardous wastes entering the water system from either disposal method is high. The use of synthetic detergents has become a common practice. Large quantities in rivers will kill fish and give amn unpleasant taste to water when it exceeds 1 pound per million gallons of water. Because detergents are resistant to bacterial decomposition, biological treatment plants cannot purify water containing detergents. New treatment processes are needed. Disposal of such waste materials as lubricating oil from a motor vehicle by simply dumping them on the ground is a common practice. While a single disposal of such waste is of minor importance, the potential of 12 million people introducing waste products into the water system becomes a major concern. Many of these practices can be altered with little effort on the part of individuals. To illustrate, old motor oil can be saved, cleaned, and used again. This simple procedure not only prevents pollution of the water system but also is a way to conserve a major resource. Highly dangerous chemicals used in agriculture as weed killers and insecticides can also pollute the water system. As much as 600 pounds of insecticides are sprayed each year on each acre of agricultural land, and consequently streams contain about 2 pounds of insecticide per million gallons of water. In addition, the effects of these insecticides on humans and fish have largely been ignored. Mineral fertilizers rich in phosphorous and nitrogen (phosphates and nitrates) are also carried into the water network and infiltrate into the groundwater. These pollutants have a number of effects on animals, humans and aquatic fauna. Domestic animals are also a source of water pollution. The washing of barns and other buildings, and water flowing from manure piles, produces a waste that enters the groundwater system as well as surface streams.
Coal Mines: Coal seams, primarily bituminous, have sulfur deposits. When these iron sulfide deposits come in contact with water, a weak sulfuric acid results. The acid drainage from both surface and underground mines has acidified thousands of miles of streams. The acid mine drainage has affected the ecology of the streams as well as the use of the water for many industrial purposes. Although techniques to purify the streams are available, the processes are costly and the problem of acid mine drainage persists.
Oil Wells: Exploitation of the petroleum resources has presented a problem of pollution from both groundwater and surface water. In the production of a single barrel of oil, as much as twenty barrels or more of saltwater are produced. Disposal of the saltwater presents a major problem to oil producers. In addition, since oil was first discovered, thousands of wells have been depleted. For decades, when the casing and tubing were pulled out of the well, the saltwater from the oil horizon was free to move into the freshwater horizons near the surface polluting them. While all oil wells must now be plugged to prevent pollution of the clean water aquifers, the plugging of the well is not always satisfactory and pollution of clean water may occur. (2.)

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