Environmental Hydrogeology
Environmental Hydrogeology
In the subsurface, three distinct phases are commonly present: liquids (predominantly water and sometimes other liquids),
gasses (predominantly air and sometimes other gasses), and solid (soil and rock matrix). Subsurface processes include
the movement of fluids in soils and rocks, movement of substances dissolved in these fluids, rock-fluid interactions,
and biogeochemical interactions.
Until a few decades ago, the major issue of interest in subsurface hydrology was groundwater exploration and supply.
The subject matter was subdivided into geohydrology and hydrogeology. Geohydrology, as the name suggests, used
to deal with subterranean hydrology. It related to the hydraulics of groundwater flow and solute transport.
Hydrogeology, as the name suggests, used to deal with the geologic aspects of water (even including surface water).
It regarded the ‘study of groundwater with particular emphasis given to its chemistry, mode of migration, and relation
to the geologic environment.’ (quoted from Hydrogeology, by S.N. Davis and R.J.M. DeWeist, 1966).
In the course of the second half of the last century, hydrogeologists and geohydrologists have broadened their scope
of interest and the distinction between the two fields has almost disappeared. In particular, their focus has shifted
from water quantity to water quality (both chemical and biological aspects). Fifty years ago, salt-water intrusion was
perhaps the only important environmental research issue in subsurface hydrology. Until a couple of decades ago,
groundwater curricula at universities emphasized topics relevant to water supply, well hydraulics and pumping tests.
New developments, however, have greatly broadened the scope of problems of interest in hydrogeology. Hydrogeology may
be loosely divided into two main areas:
Groundwater hydrogeology is the traditional area of hydrogeology that deals with the exploration and production of
groundwater for domestic, industrial, or agricultural uses. It mainly regards quantitative processes in the saturated
zone that are relatively well understood. Quality issues often relate to less complicated situations where chemical
reactions and/or degradation processes do not play a role. There are many robust groundwater models that are widely
used. Research issues concern surface water-groundwater interactions, groundwater recharge, salt water intrusion,
delineation of groundwater protection zones, regional groundwater modeling, well clogging, etc..
Contaminant hydrogeology is the study of movement and fate of pollutants in the subsurface and the methodologies
for soil and groundwater remediation and containment of pollutants. This is an extremely important field because
of social concerns about our soil and groundwater resources that are constantly threatened by human activities (urban,
industrial, agricultural, development of infrastructure). There is a clear need for cleaning and/or protecting these
resources. In fact, due to the steady rise in population, there is an increasing demand for clean groundwater supplies.
During the second half of the last century, billions of euros have been spent on the study of soil and groundwater
contamination and the remediation of severe cases. Despite the implementation of many remediation projects and
introduction of various preventive measures and regulatory legislation, major soil and groundwater pollution problems
still exits. Even if one decides not to remediate all contaminated sites, their monitoring and containment shall
remain important activities for many decades to come. Moreover, there are new activities related to the use of the
subsurface for the sustainable management of the environment and sustainable urban development. These include
(seasonal) storage of thermal energy, safe storage of hazardous waste, sequestration of CO2, and the use of the
subsurface for various underground structures. In order to ensure that in trying to solve one problem, we do not
create new problems, it is of vital importance to have (very) good knowledge of processes that play a role in such
activities and to be able to quantify consequences of our actions.
Processes of interest in contaminant hydrogeology are divers and coupled. One must study flow of fluids (water,
air, organic liquids) and their components, interactions among themselves, and their interactions with the
soil and the subsurface ecosystem. Thus, often models of contaminant hydrogeology have to include a mix of
processes such as advection, dispersion, adsorption, dissolution, volatilization and biodegradation in highly
complex porous media (with heterogeneities, fracture systems, or micro layers). As a result, contaminant
hydrogeology has become a highly multi-disciplinary research area where exchange of expertise with geochemists,
soil biologists, soil physicists, microbiologists, petroleum engineers, mathematicians, and geophysicists is
needed.
