Environmental Site Monitoring
Groundwater contamination control through monitoring wells
Once groundwater contamination has been detected, a strategic monitoring network has to be designed in order to control the evolution of the contamination plume that might be detected within the piezometers.
Location and number of piezometers are the most important aspects in order to establish a good monitoring program and these parameters will depend on location of identified groundwater contamination sources, their extent and shape, hydrogeological and geological conditions of the site (aquifer thickness, boundary conditions, groundwater flow direction, velocity of groundwater flow, etc.), location of well-fields, etc. All data available and collected before/while site visits and during intrusive investigation of soil and groundwater contamination on site by drilling works (geotechnical reports, boring logs, chemical analyses of soil and water samples, etc.) have to be analyzed carefully and considered in order to determine optimal location and number of monitoring wells.
Monitoring wells should be concentrated in the area identified as contaminated or most likely impacted. However, a network of monitoring wells should be created in such a way to control groundwater contamination, including not only piezometers located within the contamination plume but also background soil borings, which should be located in a non contaminated area in order to identify natural concentration levels.
Generally, piezometers must be installed down gradient from the contamination sources and along groundwater flow direction. If there are any drainage systems (well-fields, water extraction, water bodies, etc.) on site, or in the vicinity of the study area, some piezometers should be installed in this direction as well. Nevertheless, some monitoring wells have to be located upgradient of the contamination sources and within “clean areas” of the site in order to serve as control measurements. In some cases, already existing wells (e.g. drilled for intrusive site investigation and used for water sampling) can be used as monitoring wells.
The monitoring program should include the following activities:
- Groundwater level measurements. Static water level has to be determined and recorded in each monitoring well. Measurement of water level should be performed prior to well purging and sampling. It is recommended to use an electronic water level sensor or an oil/water interface meter.
- Collection of water samples. Water samples should be collected by bailers or disposable submersible pumps and placed directly into appropriate containers provided by a nominated laboratory, fulfilling required preservation measures, filling method and holding time. The sample containers should be appropriately labeled and kept at low temperatures during storage and transportation to the laboratory.
- Analyses of water samples including:
- Chemical substances which describe the general groundwater composition (such as Ca2+, Mg2+, Na+, K+, Cl-, SO42-, NO2-,NO3-, NH4+, HCO3-- ions, Fe2+/Fe3+, total dissolved solids (TDS), pH, biological and chemical oxygen demand (BOD/COD), etc.).
- Site specific chemical substances (Polycyclic Aromatic Hydrocarbons (PAH), Total Petroleum Hydrocarbons (TPH), organochlorinated pesticides (aldrin, endrin, dieldrin, chlordane, Pentachlorophenol, Endosulfan, Hexachlorociclohexane, etc.), polychlorinated byphenils (PCB’s), chlorinated solvents, BTEX, dioxins and furans, chlorobenzenes, organomercuric compounds, phtalates, pentachlorophenol, polychlorinated paraffins, heavy metals, etc.).
Regarding monitoring frequencies, the following recommendations might be useful:
- Groundwater level measurements: every three months (or at least twice a year during summer and winter low-flow period).
- Water sampling and analyses: general chemical substances at least every three months and specific chemical substances at least twice a year. Nevertheless, it is important to consider that these are minimum requirements. In case of very toxic contaminants sampling and analysis of water might be performed every month. This will also depend on the expected groundwater flow velocity. In this case, a numerical modeling of groundwater flow and contaminant transport might help in order to predict not only the direction of the contamination plume spreading but also its velocity. Based on the modeling results an initially established monitoring program can be accordingly corrected.
Collected and analyzed data during groundwater monitoring along with other available geological/hydrogeological information can be used for development of numerical groundwater flow and pollutant transport models.
Modeling as a method of investigation of groundwater contamination can provide theoretically substantiated long-term forecast of groundwater quality on site, direction and velocity of contamination plume movement, etc. Besides, modeling results can be used for an appropriate correction of the monitoring network structure (including number of piezometers and their locations).