A USGS scientist prepares a tracer solution in a gas-tight bladder that will be pumped into a zone of nitrate-containing groundwater to monitor the production and consumption of nitric oxide dissolved in groundwater. USGS scientists and their colleagues have developed a method to measure the rates at which inorganic nitrogen compounds, such as nitrate and nitrite, transform in groundwater.

Single-well tracer tests involve injecting a tracer solution into one port of a multilevel sampling well, creating a tracer cloud in the groundwater, and then monitoring the water chemistry in the tracer cloud from the same well as the tracer cloud moves away from the well. Single-well tracer tests can be used to measure chemical reactions in the subsurface such as measuring how nitrogen transforms in groundwater.

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.

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.

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 sewage treatment plant on the Massachusetts Military Reservation, Cape Cod discharged its treated wastewater into a series of infiltration beds through pipes like the one in the photo. This practice lasted for more than 60 years, and created a plume of wastewater more than 6 kilometers (approximately 4 miles) long in the subsurface.

USGS scientist collecting water-quality samples for the investigation of the natural restoration of the wastewater plume on Cape Cod, Massachusetts.

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.

Treated wastewater disposal beds on Cape Cod, Massachusetts, which created a large subsurface plume of contaminated groundwater. A team of scientists has been conducting long-term multidisciplinary research on the physical, chemical, and biological processes that control the transport of contaminants in groundwater.

A view of the side of a trench cut into the Cape Cod aquifer showing what is commonly referred to as a "homogeneous" aquifer. Studies of the distribution of the horizontal conductivity resulted in a range of conductivity from 0.02 to 0.34 centimeters per second, which demonstrated that the aquifer is not homogeneous.

Multilevel monitoring wells being prepared for installation prior to a large-scale natural-gradient tracer test above a plume of sewage-contaminated groundwater. Each well has 15 to 20 monitoring ports.

An array of several hundred multilevel wells were installed in an abandoned gravel pit. The array of wells was used to conduct a natural-gradient tracer test. The results of the test provided information on how contaminants are transported in groundwater.