Groundwater pathways

System considers surface water and groundwater pathways of exposure in evaluating the potential for adverse effects. Air and soil pathways will be added as will numerous built-in error checking routines. 

Investigations regarding the soil - plant pathway, and the soil -groundwater pathway, exploratory investigations... 

The surface to groundwater pathway is probably the most important way in which contaminants were initially dispersed into the local environment. Groundwater was contaminated at the plant site by infiltrating surface water and plant effluents, infiltration through soil deposits and unsaturated bedrock, and recharge to the water table. 

In the Ordinance mentioned above, control values for the soil-groundwater pathway are also included. These are evaluated at the groundwater surface. Investigations with... 

Fig. 1. Schematic diagram showing the conceptual model of the groundwater pathway in the BVG. Radionuclides are transported through the fracture network by advection with the groundwater. Two mechanisms retard migration relative to the flowing groundwater sorption and rock-matrix diffusion.

Baker, A. J., Jackson, C. P., Sinclair, J. E., Thorne, M. C. Wisbey, S. J. 1995. Nirex 95 Preliminary analysis of the groundwater pathway for a deep repository at Sellafield Vol. 3 Calculations of risk. Nirex Science Report S/95/012. 

Eor pesticides to leach to groundwater, it may be necessary for preferential flow through macropores to dominate the sorption processes that control pesticide leaching to groundwater. Several studies have demonstrated that large continuous macropores exist in soil and provide pathways for rapid movement of water solutes. Increased permeabiUty, percolation, and solute transport can result from increased porosity, especially in no-tiUage systems where pore stmcture is stiU intact at the soil surface (70). Plant roots are important in creation and stabilization of soil macropores (71). 

Multiple pathways are a major concern since depostion of PIC would have occurred. Specific soil conditions determine attenuation rates of penta PIC leachate. Once penta reaches the water table, other transport and fate processes become important. Penta exists in two forms ionized and non-ionized. The ionized form is soluble in water, while the non-ionized form is not. The ratio of the two forms in water is dependent on the pH of the aquifer. In alkaline environments penta PIC tend to be more soluble and more susceptible to advective transport and biological decay. Half-lives of penta leachate in groundwater have been estimated ranging from 27 days to 58 years. 

Generally, the main pathways of exposure considered in tliis step are atmospheric surface and groundwater transport, ingestion of toxic materials that luu c passed tlu-ough the aquatic and tcncstrial food chain, and dermal absorption. Once an exposure assessment determines the quantity of a chemical with which human populations nniy come in contact, the information can be combined with toxicity data (from the hazard identification process) to estimate potential health risks." The primary purpose of an exposure assessment is to... 

Fig. 6-7 Vertical cross-section showing pathways for surface and subsurface runoff. Path 1 HOF path 2 groundwater flow path 3 subsurface stormflow path 4 SOF. (From Dunne and Leopold, 1978.)...

Van der Meer JR, C Werlen, SF Nishino, JC Spain (1998) Evolution of a pathway for chlorobenzene metabolism leads to natural attenuation in contaminated groundwater. Appl Environ Microbiol 64 4185-4193. 

Benzene is one of a group of related aromatic monocyclic hydrocarbons (BTEX—benzene, toluene, ethylbenzene, and xylene), and since these are water soluble, there has been concern for their dissipation and persistence in groundwater under both aerobic and anaerobic conditions. Although aerobic growth at the expense of benzene was established many years ago, the pathway for its degradation was established only much later. The aerobic degradation of benzene by bacteria is... 

It is very seldom that only a single substrate is present. It is therefore important to examine how the regulation of degradative pathways may be affected and, in particular, whether the simultaneous presence of other contaminants has an adverse effect. In addition, some of the components of a contaminant may directly inhibit degradation by toxification of the relevant organism. The example of azaarenes in groundwater at a wood preservation site that inhibit PAH degradation (Lantz et al. 1997) is noted in Chapter 14. 

Blast-enhanced fracturing uses explosives, such as dynamite, to fracture rock. The explosives are placed in holes and detonated. The main purpose is to create more pathways for polluted groundwater to reach wells drilled for pump-and-treat cleanup. 

The exposure pathways of concern identified during the baseline risk assessment include direct contact, with the possible ingestion of contaminated soil (1 x 10 3 4 associated excess cancer risk), and potential ingestion of contaminated groundwater in the future through existing or newly installed offsite wells (2 x 11 0 2 associated excess cancer risk). 

---