WHAT WAS MEASURED...

Nitrate is the primary form of nitrogen dissolved in streams and ground water. In this report, nitrate refers to the sum of nitrate plus nitrite, as reported by the USGS laboratory. Nitrite concentrations commonly were less than the laboratory detection level of 0.01 milligrams per liter (mg/L), making its contribution to nitrate plus nitrite negligible.

Ammonia is a dissolved form of nitrogen that is less common than nitrate. As measured by the USGS laboratory, total ammonia includes ammonium ion and un-ionized ammonia. The latter is usually a minor component of ammonia at pHs commonly observed in streams and ground water.

Total nitrogen includes nitrate, nitrite, ammonia, and organic nitrogen. Nitrite is generally unstable in surface water and contributes little to the total nitrogen. Organic nitrogen (mostly from plant material or organic contaminants) can exist in considerable proportions and contribute substantially to total nitrogen in streams.

Phosphates are the most common forms of phosphorus found in natural waters. Compared to nitrate, phosphates dissolve less readily. They are not mobile in soil water and ground water because they tend to attach to soil and aquifer particles. They can have a significant impact, however, because eroded soil can transport considerable amounts of attached phosphates to streams and lakes. Orthophosphate typically constitutes the majority of dissolved phosphates, which can be readily assimilated by aquatic plants and promote eutrophication.

Total phosphorus includes p

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hosphates, as well as all other phosphorus forms. Dissolved phosphates and particulate organic phosphorus (mostly from plant material) are the main components of total phosphorus.

Background concentrations of nutrients are low in streams and ground water

Background concentrations of nutrients were estimated on the basis of samples collected from undeveloped areas considered to be minimally affected by agriculture, urbanization, and associated land uses. Background concentrations in undeveloped areas are controlled primarily by naturally occurring minerals and by biological activity in soil and streambed sediment. Chemical properties of the atmosphere and rainwater, which can reflect human-related fuel combustion and other activities both within and external to a watershed, can increase background concentrations. In this report, national background nutrient concentrations include atmospheric contributions and are summarized in the following table. Waters with concentrations of nutrients greater than the national background concentrations are considered to have been affected by human activities in a variety of land-use settings.

Background nutrient concentrations can vary considerably from region to region, or even within watersheds, because of differences in hydrology and in naturally occurring nutrient levels in soils, rocks, and the atmosphere. The data analyzed for this report are insufficient to define background nutrient concentrations on a regional basis. Thus, all available data from undeveloped areas were combined to derive national background concentrations. The national background concentrations are higher than most concentrations measured in relatively undeveloped areas across the Nation and may not be applicable for use in regional or local analyses.