It is common for ground water to be contaminated with the water soluble substances found in overlying soils. Many of the required data elements are similar, e.g., pH, TOC, BOD, COD, oil and grease, contaminant identification and quantification, and soil and aquifer characterization. Additional water quality monitoring data elements include hardness, ammonia, total dissolved solids, and metals content (e.g., iron, manganese). Knowledge of the site conditions and history may contribute to selecting a list of contaminants and cost-effective analytical methods.

As with soils, the pH of ground water is important in determining the applicability of many treatment processes. Often, the pH must be adjusted before or during a treatment process. Low pH can interfere with chemical reduction/oxidation processes. Extreme pH levels can limit microbial diversity and hamper the application of both in situ and aboveground applications of biological treatment. Contaminant solubility and toxicity may be affected by changes in pH. The species of metals and inorganics present are influenced by the pH of the water, as are the type of phenolic and nitrogen-containing compounds present. Processes such as carbon adsorption, ion exchange, and flocculation may be affected by pH.

E_h helps to define, with pH, the state of oxidation-reduction equilibria in aqueous wastestreams. As noted earlier in the soils section, maintaining anaerobiosis (low E_h) enhances decomposition of certain halogenated compounds.

BOD, COD, and TOC measurements in contaminated water, as in soils, provide indications of the biodegradable, chemically oxidizable, or combustible fractions of the organic contamination, respectively. These measurements are not interchangeable, although correlations may sometimes be made in order to convert the more precise TOC and/or COD measurements to estimates of BOD.

Oil and grease, even in low concentrations, may require pretreatment to prevent clogging of primary treatment systems (i.e., ion exchange resins, activated carbon systems, or other treatment system components). Oil and grease may be present in a separate phase in ground water.

Suspended solids can cause clogging of primary treatment systems and may require pretreatment of the wastestream through coagulation/sedimentation and/or filtration. Major anions (chloride, sulfate, phosphate, and nitrate) and cations (calcium, magnesium, sodium, and potassium) are important for evaluating in situ geochemical interactions, contaminant speciation, and water-bearing zone migration. Iron concentrations should be measured to determine the potential for precipitation upon aeration. Alkalinity should also be measured when analyzing for major anions and cations.

In addition to chemical parameters, geologic and hydrologic information is usually needed to plan and monitor a ground water remediation. A detailed geologic characterization is usually needed to assess the uniformity (homogeneity and isotropy) of the subsurface hydrostratigraphy. The average rate of ground water flow can be estimated from the hydraulic conductivity, hydraulic gradient, and effective porosity. Hydraulic gradient is calculated from ground water elevations measured in monitor wells. Effective porosity is usually assumed based on ranges of values cited in scientific literature or estimated from pumping tests. Hydraulic conductivity is usually estimated from slug tests or pumping tests. If an active ground water extraction system is being planned, safe aquifer yields and boundary conditions must be established. These parameters require that pumping tests be conducted.