The Fate & Transport of Contaminants in a Karst Aquifer
Groundwater flows just beneath our feet every day, yet few think about how important it is to everyday life. The contamination of groundwater is becoming a growing issue in today’s society. Leakage from an underground storage tank, industrial waste mismanagement, or a spill from a gasoline tanker truck can all cause contamination to groundwater. There are numerous factors that control the fate and transport of contaminants in groundwater to include but not limited to contaminant type, aquifer characteristics and local hydrogeology. Today, we’ll take a look at two of the main contaminant types that can occur from leaks and spills: dense non-aqueous phase liquids (DNAPLs) and light non-aqueous phase liquids (LNAPLs). Contaminants migrate differently through groundwater depending on the type of contaminant and the aquifer in which they migrate through.
In southeastern Pennsylvania, the dominant aquifer is comprised primarily of karst. Karst aquifers occur on bedrock made up of carbonates (limestone or dolomite) and are characterized by conduits and cavities that have been created or enlarged by dissolution of the carbonate. The conduits and cavities both store groundwater and provide paths for the groundwater to migrate through the subsurface. Groundwater flows slowly from higher hydraulic pressure to lower hydraulic pressure via the cavities and pores in the limestone. The path of groundwater flow often follows surface topography; ultimately leading the groundwater to some discharge point into a stream, river, or some other body of water. Bedrock structure, fractures and faulting, preferential pathways, and confining layers can also control groundwater flow direction.
The Fate and Transport of Substances that Sink
Dense Non-Aqueous Phase Liquids (DNAPLs) have a density that is less than the density of water and therefore will mix or sink when they encounter water and may not readily dissolve in their current state. This is similar to adding sugar to water, the sugar will sink to the bottom, but will not easily dissolve immediately. It usually takes some sort of catalyst to dissolve the sugar completely. DNAPLs include chlorinated compounds such as trichloroethylene (TCE) and tetrachloroethylene (PCE), which are widely used in the dry cleaning industry and as a degreasing chemical in manufacturing processes. The transport of DNAPLs can occur in two different ways, both of which begin with infiltration of contaminants into the vadose zone. The vadose zone is also known as the unsaturated zone, where the void and pore spaces are partially filled with water and air. There the contamination can either be exhausted by the residual saturation in the vadose zone or can infiltrate the saturated zone directly.
The first case involves DNAPLs being exhausted in the vadose zone by the residual saturation. Once the contamination is impeded in the vadose zone it can still reach the water table by numerous processes and result in groundwater contamination; just because it was exhausted does not mean it was stopped. The DNAPL can be present in the vadose zone in various physical states (air, liquid, solid, and immiscible phases). The physical states may occur all at the same time, leading to a very complex system, or in any other combination. The most common ways for the contaminant to reach the saturated zone is via the air and liquid phase. DNAPLs can volatize into a gaseous state which can migrate as a vapor plume to the water table and into groundwater. The DNAPLs can also dissolve and travel as soluble particles into the groundwater dependent upon their solubility value.
The vadose zone in the second case does not have enough residual saturation to exhaust whatever volume of DNAPL has infiltrated the surface. The DNAPL will continue to infiltrate downward to the water table and then infiltrate the groundwater flow directly. The high density of the DNAPL will cause it to continue sinking until the volume is dissipated by the residual saturation process or until it reaches a substance of low permeability and begins to pool or move laterally in the direction of groundwater flow. The DNAPLs will continue to be transported with the groundwater flow until a discharge point is reached or the contamination is contained.
The Fate and Transport of Substances that Float
Light non-aqueous phase liquids (LNAPLs) have a density less than that of water and therefore will float when introduced to water. LNAPLs include common petroleum products such as diesel fuel, gasoline, and jet fuel. All of these products are immiscible, meaning they will not mix with water in their current state. LNAPLs are similar to olive oil in a pot of water, the oil will not sink, it just sits on the surface of the water. The contamination once again begins in the vadose zone where the LNAPLs begin to migrate downward slowly, solely due to the force of gravity. In many cases, LNAPLs may be contained in pores as residual saturation located in the vadose zone. Downward infiltration of water from recharge zones may eventually dissolve soluble particles contained in the pores and mix with the groundwater, resulting in a liquid-phase contamination plume. The LANPLs may also volatize and become an air phase contamination plume, also known as a vapor intrusion. The liquid and air phase contamination clouds then migrate towards the water table and into groundwater (Newell, Acree, Ross, and Huling, 1995)2. Once the liquid and air phase plumes infiltrate the groundwater, they will both travel in the direction of the groundwater flow until they reach a discharge area.
As the LNAPLs travel along the groundwater flow they can encounter barriers and begin to buildup. This is a potential hazard because the buildup of a vapor plume can be released into caves that tend to occur with karst aquifers and bedrocks. Most of the gases are combustible and can be dangerous in large quantities to cavers. There also tends to be a large amount of the liquid phase contamination plume left behind in any water contained within the cave. There have been cases in which 1-2 inch thick layers of gasoline or diesel fuel appear on the surface of lakes and other bodies of water miles from a spill or leakage site after being transported by groundwater flow.
The contamination of groundwater is a serious problem that doesn’t always receive the attention necessary to address it until it has impacted a community. A large majority of the United States population relies on groundwater as a safe, drinkable source of water. Groundwater is used in just about every aspect of life to include industrial, agricultural, and commercial purposes. All groundwater comes from the water cycle, which is a closed system, so once it is gone or contaminated there won’t be new water to simply replace it. The process of cleaning contaminated groundwater can be very costly, which is why it is best to be proactive when it comes to groundwater contamination. To protect the community at large spills and groundwater contamination should be addressed and contained as quickly as possible following the release.