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.

A close-up of the ports of a multilevel well that's part of the large-scale tracer-test array at the site. Each port is covered with a nylon mesh.

Color-coded tubes sticking out of the top of a multilevel monitoring well. Each tube is connected to a port on the side of the well.

Winter-time view of multilevel well sampling array on Cape Cod, Massachusetts. There are over 10,000 subsurface sampling ports. The array is use to conduct natural-gradient tracer tests that are designed to test hypotheses about the transport of contaminants in the subsurface.

A tracer solution (bromide, lithium, fluoride, and molybdate) was injected into three wells at the start of a large-scale (280 meters) natural-gradient tracer test (circa 1985 to 1986).

A special sampling apparatus was designed to collect water samples from multi-level monitoring wells during large-scale natural-gradient tracer tests. Each well has up to 15 sampling ports.

During the first large-scale natural-gradient tracer test conducted at the site, wells from the array of over 600 multilevel wells were sampled about once a month (circa 1985 to 1986).

USGS scientists monitored the first large-scale natural-gradient tracer test conducted at the site for over 17 months (circa 1985 to 1986).