Geochemical Survey of Iowa

The United States Geological Survey (USGS), in collaboration with other state and federal agencies, industry, and academia, is conducting a National Geochemical Survey (NGS) to produce a database of geochemical information for the United States based primarily on stream sediments, analyzed using a single set of methods. This data set will comprise a national-scale geochemical coverage of the US, and will enable construction of geochemical maps, refine estimates of baseline chemical element concentrations in the sampled media, and provide context for a wide variety of geological and environmental studies. The goal of the NGS is to analyze at least one stream-sediment sample within every 289 km2 area across the US, using a consistent set of analytical methods, substituting soil samples where necessary. The survey incorporates geochemical data from a variety of sources, including existing analyses in USGS databases, reanalyses of samples in USGS archives, and analyses of recently collected samples. Currently, the NGS data covers ~71% of the land area of the US, and includes samples from all 50 states. The Iowa Geological Survey (IGS) of the Iowa Department of Natural Resources (IDNR) entered into an agreement with the USGS in September of 2002 whereby the IGS would design a database for field parameters and collect two soil sample sets. One set for the USGS to process and analyze, and one set to reposit. Field parameters included ambient site conditions, GPS location, elevation, landscape position, vegetation type, NRCS soil type, sample depth, and soil horizon, texture, color and moisture. Digital photographs were taken of the site and samples at each location. The samples were sieved to -100 mesh, then analyzed for 40 elements using inductively coupled plasma-atomic emission spectrometry/acid dissolution (ICP40), and 6 elements using atomic absorption spectrometry (AA). Arsenic (As) and gold (Au) were analyzed for using both methods. Analyses were performed by the USGS or an approved laboratory, using standard methods and an U.S.E.P.A. approved quality assurance/quality control plan. To maximize statistical reliability, sample collection in Iowa was based on a 17 km x 17 km grid, displayed on USGS 1:250,000 quad maps. Each grid or cell was identified by quad name and cell column and row position, and divided into four 72 km2 quadrants, and one was selected at random for sampling. The IGS selected specific sampling sites within the selected quadrants. To separate leached horizons from those accumulating CO3, one shallow (0-8 inches) and one deep (12-24 inches) sample were collected from 463 regular and 72 analysis of variance (AOV) sites from May through August of 2003 and shipped to the USGS in August. Randomly selected AOV sites were sampled to provide a data set for statistical analysis to test the adequacy of the samples to measure differences of sediment chemistry between cells, within cells, within sites, and between chemical analyses. "AOV1" was collected within the designated quadrant of the cell, then one of the three other quadrants of the cell were selected at random for "AOV2" and "AOV3" which were collected about 10 feet apart, preferably within the same soil type.