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Nutrients National Synthesis Project

Data on nutrients in the streams, rivers, and ground water of the United States

Explanation of Ground Water Data

    The National Water-Quality Assessment (NAWQA) Program analyzes water quality at the national scale using consistent well type, sampling depth, targeted land use, and laboratory analytical methods. NAWQA study units generally conduct three types of ground-water studies: (1) "land-use studies," which sample shallow ground water (just below the water table) beneath a targeted land use (such as agricultural or urban land); (2) "major aquifer surveys," which sample deeper ground water irrespective of overlying land use and commonly used for drinking; and (3) "flowpath studies" that investigate ground water-surface water interactions by sampling ground water along an individual flow path that terminates in a stream. The data provided here are from land-use and major-aquifer studies, which are designed to facilitate statistical comparison of regional water-quality conditions. This data set, identical to the one being used in the USGS Periodic National Summary, is a carefully prepared subset of the NAWQA national-synthesis database. The data subset is designed to minimize sampling bias from different sampling strategies. The data subset represents "land-use studies" and "subunit surveys" of major aquifers in study units. The following sites are excluded from this data set:

• springs and drains, because of uncertainties in the source of water and contributing land-use area;
• networks with less than 10 wells (unless in undeveloped areas, such as forest land);
• spatially co-located (nested) wells, to preclude over-sampling of an area for purposes of national comparison;
• and densely sampled networks in a small area, to preclude over-sampling.

    Concentrations are in milligrams per liter (mg/L) or micrograms per liter (µg/L). A negative value indicates that the concentration in the sample is below the value which follows, the laboratory method reporting limit, with the absolute value of this number giving the method reporting limit, and a "." in a field indicates there are no data available. The codes and their meanings for the Depth-to-Water Measurement are:

• E - The site was flowing recently.
• P - The site was being pumped.
• R - The site had been pumped recently.
• S - A nearby site that taps the same aquifer was being pumped.
• V - Foreign substance was present on the surface of the water.

    Definitions of fields in the data set are provided in the following list. "P" codes are the five-digit numbers used to identify water-quality constituents and properties in the USGS National Water Information System (NWIS) and the U.S. Environmental Protection Agency Data Storage and Retrieval System (STORET). Most "P" codes are preceded by a remark field which contains important information about that measurement. A "<" indicates that the concentration in the sample is below the value which follows, the laboratory method reporting limit, a concentration preceded by "E" indicates an estimated value, and a concentration preceded by ">" indicates a value "greater than" the maximum calibration value for the method.

    The Ground Water Site Description table gives precise information (latitude and longitude in NAD83) about the geographic location of each site.

NUTRIENTS IN GROUND WATER

GROUND WATER SITE DESCRIPTION