Understanding subsurface fate and transport draws on all of the elements of Environmental Geology we have been looking at this far. The brief summary below, though not all-inclusive, provides an excellent illustration of the fact that all aspects of the environment and our human relationship to it are interdependent.  Nothing occurs in isolation on earth, and each concept you have learned builds from the previous one. You may want to review material from previous modules.

LIST OF FACTORS - AQUIFER CHARACTERISTICS:

A. AQUIFER STRUCTURE

Determining the overall structure of the aquifer is extremely important. Is the aquifer heterogeneous or homogeneous? What are the dominant mineral  types? What is the distribution range of grain sizes? Are there any features that could provide preferential flowpaths (such as fracture zones, or well shafts)? These questions and others must be addressed.

Actual amount of organic carbon present in the soil matrix. A higher percentage of organic favors partitioning of hydrophobic compounds into the solid phase.

The dry mass of a unit mass of soil per unit volume of soil. A typical range being 1.2 g/ml for fine clay to 1.7 g/ml for sand.

This is reported as a percentage, and is necessary for the calculation of free pollutant volume. Porosity is the amount of void space in a given material. Subsurface n occurs in two forms where primary porosity represents the intergranular component, and secondary porosity represents n resulting from fractures, fissures and the like. Calculated porosity (effective or drainable porosity) will be lower than actual porosity as not all void spaces will be connected and so not all will drain. Decreasing porosity generally translates to decreased permeability and hence pollutant migration will be reduced.

This factor is what determines the rate at which contaminants may leach through the subsurface. Only pore spaces that are completely connected will permit pollutant flow. Permeability correlates to the drainable porosity described above, as well as the specific yield described below. Measured as an area in ft² or in².

F. SPECIFIC YIELD (Sy)

Amount of moisture that a soil will yield under gravity drainage. This is the volume of contaminated soil water that may be available to leach contaminants through the vadose zone.

Volume of water that a soil retains against gravity drainage. Correlates to water holding capacity.

This represents the maximum volume of water a soil contains that may be available to support biological activities after leaching and throughflow. The optimal WHC for maximum aerobic microbial activity ranges from 60-80% of total soil volume. WHC is also the same as saturation limit.

Saturation is defined as the amount of fluid contained within the pore spaces of the vadose zone (WHC). Soil saturation limit refers to the relative amount of liquid free product in void spaces compared to the total amount (WHC) possible. Maximum hydrocarbon retention capacity is generally 2/3 WHC for liquids (vadose soils may be completely saturated with gasoline).

This is measured in units of length/time such as m/s and represents the ability of a material to permit the passage of fluid through its interior. Hydraulic conductivity is a calculated value (i.e. from Darcy's Law, discharge, or other measured value).

This is a measurement of fluid potential and can be thought of as the gravitational potential energy inherent in standing well water rising to a certain elevation in relation to a given reference point. Hydraulic head is the driving force for the flow of groundwater, and flow always occurs from an area of high h to one of Iov~~er h. Hydraulic head thus determines the hydraulic gradient of a given aquifer.

Described by h₁ - h₂/ L where h₁ is the hydraulic head at reference well 1, h₂ is the hydraulic head at reference well 2 and L is the distance between them. This is a unitless value and represents the hypothetical slope of the water table, indicating direction of groundwater flow. The zero elevation reference point is normally taken at sea level, and hydraulic head data are reported as elevation above mean sea level (amsl).

This is the cross-sectional area of the aquifer of interest using width and thickness as the multipliers.

This is the volume of water flowing through an aquifer per unit time.

Transmissivity is the rate of flow Occurring through a unit thickness of the saturated aquifer under a unit hydraulic gradient (I = 1.0). Transmissivity is calculated by the formula T = kb Where k is the hydraulic conductivity and b is the saturated thickness of the aquifer (b = 1.0 by definition).

Oxygen serves as the terminal electron acceptor for the aerobic degradation of organic constituents and thus is of major importance. Oxygen will be present in a separate gas phase or dissolved in soil water. Oxygen must first pass into the aqueous phase before it is useable by microorganisms. Frequently, dissolved oxygen concentration is the rate-limiting factor controlling enzymatic degradation of organic compounds in the subsurface.

Biochemical Oxygen Demand is defined as the amount of oxygen that is required for the complete, aerobic degradation of organic compounds to carbon dioxide and water. Both abiotic and biotic reactions will consume oxygen. If the natural BOD of the subsurface soils and waters are high, the partial pressure of oxygen in the, subsurface may be insufficient to support hearty microbial action on the contaminants of concern. Addition of oxygen would therefore be necessary if in situ biological treatment is used. An overall BOD can be estimated by measuring metabolism of a microbial population displaying first order degradation kinetics. A five day measurement is taken to get an average value (BOD₅) based upon oxygen Consumption where BOD₅=BODu[1 - exp(-kt)] and t = 5 days. BOD is reported as the grams of oxygen required to degrade the organic species in 1 kilogram of soil or liter of groundwater. BOD for particular constituents can then be inferred.

C. pH

This is the inverse log of the concentration (activity) of free hydrogen ions (protons) in a given area. pH=-log[H+]. The pH strength of a solution is measured on a scale ranging from 1.0 (most acidic) to 14.0 (most basic). The pH concept is illustrated by the simple dissociation of the water molecule which has a slightly acidic character.

H₂0 <====> H+ + OH^-

The dissociation constant governing the equilibrium is KH₂O = [H+][OH^-] = 1 x 10-14. Neutral water will have a pH of 7.0 where [H+] = [OH^-] = 1 x 10^-7. Optimum pH range for microbial activity is between 6.0 and 8.0.

Interfaces are areas where abrupt transitions from one set of environmental conditions to another suddenly occur. Examples include sudden changes in pH or dissolved oxygen concentration in the transition from water to sediment, or different phases of a compound (or multiple compounds) that are in equilibrium with one another - such as liquid-vapor or solid-vapor molecular mass transfer equilibria. The diffusion and biodegradation rates of pollutants are limited by the maximum rates at which these transitions can occur. Phase interface data is measured as surface tension and reported in dynes/cm.  Interfacial tension has a major effect on the wettability of a soil. Higher surface tension translates to a stronger capillary action (i.e. more rapid travel through the subsurface). Oxygen interfaces result in areas of high oxidation/reduction (redox) potential. Redox is a measure of the relative tendency of a molecule or solution to accept or transfer electrons.

Frequently transitions in hydrogen ion concentration (pH), oxygen levels (Eh), surface tension or other factors occur in the region approaching an interface. The development of a smooth transition zone, or gradient, may occur on both sides of the interface - both in terms of contaminant concentrations and oxidation/reduction (redox) potential.

Interfaces and gradients may support microenvironments where maximum biodegradation can occur. In general nutrient concentrations tend to be elevated in these areas and the interfacial surfaces may provide areas where microbial attachment and possible biofilm formation can occur. The existence of such transitions may indicate favorable biodegradation conditions, and thus contaminant migration may be slower than expected. Redox gradients may also enhance metabolic activity.

Carbon dioxide concentration has a significant impact on all environmental phases. In soils and groundwater it can contribute to acid-base equilibria. C0₂ concentration can be used as an indicator of soil and water quality as it is a necessary nutrient for autotrophic microbes and plays a major role in determining microenvironment conditions. The carbonic acid equilibrium and the alkalinity of the receiving environment are used to determine carbon dioxide concentration and phase distribution, as well as provide an indication of biological activity.

Aqueous carbon dioxide is part of an equilibrium system of three species - carbon dioxide (as carbonic acid), bicarbonate ion, and carbonate ion. These reactions occur in the pore water of the vadose zone and in the aquifer. The system can be described by the following equations:

Where pKa is the log of the acid deprotonation equilibrium rate constant.

Alkalinity is a capacity factor that expresses the ability of groundwater or soil to neutralize free H+ ions. By contrast, bascicity is an intensity factor that describes the concentration level of free hydroxide anions (OH-). Alkalinity is measured in equivalents (moles) per liter by the formula:

[alk] = [HCO₃] + 2[CO₃-²] + [OH]

Average alkalinity of waters is 1 .00 x 10-3 eq/L or 1 x 10-3 moles of H+ ions neutralized per liter of water.

2. Total Dissolved Solids I Total Suspended Solids

Water analyses for solids require that concentrations of cations and anions be charge balanced. Calculations are based upon equivalents (moles) of charge per liter of solution. Electrical conductance and nutrient concentrations are correlated to TDS and TSS.

This is the ability of a particular soil to retain cations. CEC depends on soil type and increases as the proportion of clays or concentration of humic substances increases. CEC is reported as miliequivalents (milimoles) of charge per 100 grams of dried solid. Many of the exchangeable ions are micronutrients required for cell growth and metabolism; thus cation exchange capacity can also be interpreted as nutrient status.