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Natural gas industry sites have been contaminated in the past by elemental mercury (Hg) from gas flowmeter manometers. Flowmeters (metering stations) are located throughout the natural gas distribution system at wellheads, at gas processing plants, along gas transmission lines, at underground gas storage facilities, and at points of end use. Field site instrumentation has occurred at six field research sites located in natural gas production areas of the United States. These field research sites, located in Louisiana, Pennsylvania, and New Mexico, are representative of diverse climatic, geologic, and hydrologic conditions. Irt situ hydraulic conductivity measurements on these sites represent a range of 10(4) to an estimated 10(-10) m/s. Mean annual precipitation ranges from near desert (extremely negative annual water budget) to subtropical (extremely positive annual water budget). Geologic materials found on the six sites include stratified alluvial clays, weathered bedrock, and coarse sands and gravels. Each of these sites has had documented spills of elemental Hg and has been instrumented with stainless steel monitoring wells, each of which has a dedicated stainless steel and Teflon(R) sampling pump. These monitoring points have been sampled quarterly in an effort to develop fundamental information on the transport and fate of Hg in the shallow subsurface, under a range of climatic, geologic, and hydrologic conditions. Both field-filtered and unfiltered groundwater samples have been collected as part of the quarterly sampling effort to determine the potential role of colloid-borne Hg transport in shallow groundwater systems. Data collected from five consecutive quarterly sampling efforts at the New Mexico and Pennsylvania sites suggest that there has been no apparent adverse impact to shallow groundwater in the immediate vicinity of the research sites. A quarterly monitoring program is currently under way at the two Louisiana sites.