Dissolved oxygen is found in groundwater more often than one might expect. If water recharging an aquifer comes into contact with the atmosphere first, dissolved oxygen can enter the water and remain there.


In groundwater sampling, monitoring DO is one way to determine when conditions have stabilized. Water in the well screen can possess different characteristics than the aquifer surrounding it. When dissolved oxygen levels stop changing during a well purge, it’s a sign that the water sample is representative of the aquifer.


Similarly, dissolved oxygen can play a role in evaluating well construction. As with a well purge, stabilized DO indicates the well is bringing in water from the aquifer and will function as intended.


Depending on the contaminant present, oxygenating groundwater can be an important step in the remediation process. In some cases, remediation involves adding microbes to groundwater to consume the contaminants. Adding dissolved oxygen is essential for this strategy to work. DO feeds the microbes; with adequate DO, microbial activity and numbers increase and they’re able to process more contaminants.

Dissolved oxygen plays a crucial role in wastewater treatment as well. Microbes are an essential part of this process, as they break down contaminants in the water. Treatment plants use aerators to “activate” the sludge, pumping dissolved oxygen into the contaminated water so aerobic bacteria have the resources they need to grow and reproduce. The bacteria help prepare the water for the next stage in the treatment process by decomposing organic material and changing the chemical makeup of the sludge through nitrification.


Treatment plants must meet regulatory requirements when discharging treated wastewater, or effluent, back into the environment. These requirements are in place to ensure effluent contains a healthy level of nutrients and doesn’t create plant overgrowth or hypoxia after discharge. Nutrient removal processes also require the presence of microbes and bacteria to control levels of nitrogen, phosphorous and other nutrients, and dissolved oxygen is an important factor in creating optimal conditions for these organisms.

Colorimetric Method

In the Colorimetric Method, a reagent is combined with the sample to give the solution a color. The resulting color will be more vibrant if the solution contains a higher concentration of the selected parameter. A wavelength of light is then chosen based on what the parameter will absorb. The amount of the parameter present can be calculated based on how much light is able to pass through the solution.

Winkler Titration Method

Electrochemical Sensors

Electrochemical sensors, also known as membrane-based or membrane-covered sensors, come in two varieties, polarographic and galvanic. The sensing element is covered by a semipermeable membrane that allows dissolved oxygen to pass through. There are two electrodes–a cathode and an anode–within the sensor, surrounded by an electrolyte solution. Though polarographic and galvanic sensors use different methods of measurement, both use the electrical properties of water to determine the concentration of dissolved oxygen.


All electrochemical sensors need to pass current to take a measurement, giving them a higher power requirement than optical sensors. They also need continual flow across the sensor face to read accurately, because they consume oxygen while operating. If deployed in small, localized areas, this could change the concentration of DO or deplete it enough that the sensor can no longer take readings. To ensure accurate measurements, electrochemical sensors require either aeration or a stirring system to circulate water around them.

 Optical Sensors

Optical sensors measure luminescence decay to determine the partial pressure of oxygen and calculate concentration of dissolved oxygen. In-Situ’s RDO sensors measure dissolved oxygen through a process called “dynamic luminescence quenching.” The sensor uses a special RDO cap with a gas-permeable sensing foil. The sensing foil contains “lumiphore” molecules which fluoresce when excited by blue light. A blue LED inside the sensor emits blue light, and the sensing foil in turn emits red photons. If oxygen is present, the oxygen molecules quench this fluorescence, so fewer red photons are emitted.

The distance between the peaks of the blue emission wavelength and the red reference wavelength is called the phase angle shift. Phase angle shift is proportional to the partial pressure of oxygen in the liquid or gas surrounding the sensor. As it measures the delay of the returned signal, it is thus based on the fluorescence “lifetime” rather than “intensity.” Therefore, DO concentration is derived from measuring the luminescence lifetime based on the phase shift between red returned light from the excited lumiphore molecules and the red reference light from the red LED.