USGS scientists processing groundwater samples during a subsurface pH modification experiment. In the foreground is a tank containing an injection solution used to create a plume of groundwater with lower pH.

USGS scientist filtering water samples for nitrogen isotope analysis during the tracer test on Sugar Creek, Indiana. Analysis of the tracer test data allowed for the direct observation of denitrification within the Creek.

Preliminary pulse-injection of rhodamine tracer moving down Sugar Creek, Indiana. This preliminary injection was used to guide the nitrogen isotope tracer test.

A conceptual diagram of the setup of the subsurface tracer test. A solution of bromide (conservative tracer), 17ß-estradiol, 4-nonylphenol, and sulfamethoxazole was injected into the subsurface. A series of corresponding water samples were collected from the multilevel sampler downgradient of the injection well.

USGS scientist preparing equipment for a tracer test monitored with geophysical equipment, Charleston, S.C. The information from the tracer test is used to calculate realistic field-scale rate-limited mass transfer coefficients. The scientists use the coefficients to more accurately simulate the transport of contaminants in fractured rock.

Diagram of borehole geophysical setup for tracer-test monitoring. Three boreholes were instrumented with electrodes used to take electrical resistance measurements during the push-pull test, which was performed from a central injection/extraction borehole.

A diagram of a natural gradient, single well injection test that can be used to estimate the rate that hydrogen is consumed by bacteria in the subsurface. Step 1 of the test involves the controlled injection of a solution of dissolved hydrogen gas and a non-reactive tracer using a single port of a multilevel monitoring well. Step 2 involves the collection of water-quality samples from the plume of hydrogen and tracer as it drifts past the same well.

A diagram of a push-pull, single well injection test that can be used to estimate the rate that hydrogen is consumed by bacteria in the subsurface. Step 1 of the test involves the controlled injection of a solution of dissolved hydrogen gas and a non-reactive tracer into a monitoring well. Step 2 involves pumping the injected tracer solution out of the subsurface using the same well, and collecting water-quality samples from the pumped fluid.

Acid mine drainage from California Gulch (right) mixes with the waters of the Arkansas River, Colorado (left). The chemical reactions that occurred as water from the Gulch mixed with the River's water controlled the quality of water downstream of the confluence.

Multilevel well sampling array with over 10,000 subsurface sampling ports at the Cape Cod Site, MA. Several large-scale natural-gradient tracer tests and numerous small-scale tracer tests have been conducted with the array.

Tracer test being conducted to study subsurface contaminant transport at the Bemidji Site, MN.

USGS scientists processing water samples during a stream tracer test on California Gulch, Upper Animas River Watershed, CO.

Locating inflows and springs discharging into Mineral Creek, CO, prior to conducting a tracer test. Tracer tests are used to assess the relative magnitude of metal loading from inflows along a stream reach.

A site of continuous water-quality sampling during a tracer test conducted on the North Fork of the Animas River, CO.

View of a data logger used to control a pump that injects a tracer solution into a stream during a stream tracer test.

USGS equipment trailer -- bringing tracer tests to Rocky Mountain streams.

Spacing sampling points along a stream prior to a tracer test on Mineral Creek, CO.

Filling a holding tank with a tracer solution prior to the start of a stream tracer study, Upper Animas River Watershed, CO.

View of the setup of a NaCl tracer test that was designed to determine natural attenuation of metals in Pinal Creek, AZ.

Downstream view of Pinal Creek, AZ, where a tracer solution was injected into the stream during a tracer test. Water-quality sampling crews can be seen in the distance.