U.S. Geological Survey (USGS
) scientists prepare a tracer solution to conduct a natural gradient tracer test to measure anammox activity in groundwater
. The tracer solution is prepared by pumping groundwater into a gas impermeable bladder to maintain the in situ dissolved oxygen concentration and then mixing in the tracer chemicals. The bladder is contained in an ice-cooled water pool to maintain the groundwater temperature until the solution is pumped back into the aquifer. Photo Credit: Deborah Repert, USGS
A diagram of the natural gradient tracer test used to detect anammox in groundwater
. The tracer solution was injected in the well on the left, and wells on the right were used to monitor the subsurface plume caused by the tracer injection. Analytical testing for the isotopic ratios of nitrogen gas (30
) helped determine rates for the denitrification of nitrate (NO3-
) to nitrogen gas (N2
) and for anammox of ammonium (New Hampshire4+
) to N2
. From Smith and others, 2015
Experimental setup for small-scale transport test designed to assess the effectiveness of the "colmation layer" at the bottom sediments of Ashumet Pond (Cape Cod, Massachusetts) for filtering out cyanobacteria and viruses
. The green color in the collapsible bag containing the colloids and a bromide tracer is caused by the presence of microbial sized fluorescent microspheres that scientists added to act as colloidal tracers. Photo Credit: Denis R. LeBlanc, USGS
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
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 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 a 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).
scientists monitored the first large-scale natural-gradient tracer test conducted at the site for over 17 months (circa 1985 to 1986).
The thousands of sampling ports in the subsurface sampling array used for the large-scale, natural-gradient tracer test created a mind-boggling number of water samples. In later stages of the test over 4,000 samples were collected during sampling field trips (circa 1985 to 1986).
Areal view of the sewage disposal beds and the gravel pit with the tracer test sampling array on Cape Cod, Massachusetts. Ashumet Pond is to the right, and the disposal beds are on the upper left. The disposal beds are the source of a subsurface plume of contaminants that's more than 6 kilometers long.
The gravel pit on the Massachusetts Military Reservation, Cape Cod, where USGS
and other scientists have conducted over 50 tracer tests involving reactive and non-reactive solutes, microspheres, and deactivated microorganisms.
This wintertime areal view helps illustrate the more than 1,500 wells that have been drilled at the research site on Cape Cod, Massachusetts. Thousands of water samples have been collected and analyzed to characterize a subsurface plume of sewage, and to investigate processes such as the movement of bacteria and viruses in the plume.
The sample-freezing drive shoe
and a tank of liquid carbon dioxide (CO2
) in the background. The CO2
is used to freeze the very bottom of a core while it's still in the subsurface.
Field crew working to cap the bottom of the core liner after a successful coring attempt with the sample-freezing drive shoe. The sample-freezing drive shoe enables the recovery of full length cores even in unconsolidated sandy aquifers.
Closeup of the sample-freezing drive shoe and a core in a polycarbonate plastic liner removed from the core barrel. The frozen section at the bottom of the core prevents the core sample from dropping out of the core barrel when it's pulled up to the surface.
Close up of the sample-freezing drive shoe after retrieval from a borehole. Ice has accumulated on the outside of the drive shoe as a result of liquid CO2 that was pumped down into the drive shoe while still in the borehole. Freezing the bottom of the core enhances core recovery.
A handful of sand from aquifer sediments on western Cape Cod, MA. The surfaces of these quartz grains are covered by coatings containing iron and aluminum oxides and silicates. Arsenic in the coatings can be released by changes in chemical conditions.
Electron photomicrograph of a cross section of a quartz grain from sediments on Cape Cod, MA, shows coatings (white material at the surface). The coatings contain arsenic that can be released under changing chemical conditions.
scientists conducting a tracer test where clean, oxygenated groundwater was injected into an anoxic zone beneath the sewage infiltration beds. The tracer test was part of a study of the natural restoration of a subsurface sewage plume after the use of the infiltration beds (the source of the plume) was discontinued.
Divers from the USGS
Science Center for Coastal and Marine Geology, Woods Hole, Massachusetts, assisted with the installation of wells designed to better understand the upward hydraulic gradients beneath Ashumet Pond, Cape Cod, Massachusetts.
Seepage meters were deployed to measure fluxes of water and associated phosphorus concentrations discharging into Ashumet Pond, Cape Cod, Massachusetts. A National Association of Geoscience Teachers
student intern is connecting a seepage bag to the meter.
scientists installing diffusion samplers used to monitor the performance of a reactive barrier designed to remediate a phosphate plume discharging to Ashumet Pond, Cape Cod, MA.
scientists installing an experimental horizontal well to test its usefulness for monitoring the performance of a reactive barrier that was installed to remediate a phosphate plume discharging to Ashumet Pond, Cape Cod, Massachusetts.
The sediment along the shoreline of Ashumet Pond, Cape Cod, MA, before the installation of a permeable reactive barrier to remediate a plume of sewage discharging to the pond. The black color of the sediment is the result of manganese in the plume precipitating to manganese oxide when groundwater with very little dissolved oxygen encounters the oxygen rich pond water.
Shortly following the installation of a permeable reactive barrier, the sediment along the shoreline of Ashumet Pond, Cape Cod, MA, turned red, indicating the oxidation of the iron filings in the barrier. The barrier was constructed to remediate a phosphate plume discharging to the pond.