Groundwater Contamination
Given its importance as a supply of freshwater needed to support society, the quality of groundwater is of paramount importance. Yet, modern civilizations have a profound impact on groundwater quality. From industrialization and urbanization associated with 21st-century ‘mega cities’ and population increase, to intensive farming and deforesting, there is a significant use of resources and generation of waste that make groundwater unsuitable for use. Groundwater may not be suitable for use because it contains dissolved inorganic substances coming from natural sources (e.g., chloride, sulfate, etc.) or organic liquids, dissolved organic/inorganic constituents, or pathogens from anthropogenic sources.
The purpose of this exercise is to explore variabilities in groundwater contamination that make management a challenge.
Answering the following questions requires visiting the ‘Groundwater and Society Gallery’ and the ‘Groundwater Contamination’ webpages in the magnet4water.com Digital Library ( Library > Education Videos > Groundwater Contamination). Also see the figures and text that follows.
Questions
Question 1: Discuss/summarize/categorize the major source of groundwater contamination, their distributions globally/regionally, and mechanisms for contamination (how does the source get into groundwater). Provide some examples of particular importance/relevance in your country, state, province, city or local area.
Question 2: Explain why political boundaries are often irrelevant for the management of contaminated groundwater.
Question 3: Discuss why groundwater contamination must be studied with a system-based approach. In other words, explain how processes/mechanisms at different scales of a water systems impact groundwater contamination. Consider how human activities exacerbate contamination or interlock with natural processes at different scales to impact groundwater quality.
Figure 1: Proportion of population using improved drinking water sources (%). From the World Health Organization (2014), Health Statistics and Information Systems (HIS).
Natural Sources
Arsenic
Arsenic occurs naturally as a trace component in many rocks and sediments. Whether the arsenic is released from these geologic sources into groundwater depends on the chemical form of the arsenic, the geochemical conditions in the aquifer, and the biogeochemical processes that occur. Arsenic also can be released into groundwater as a result of human activities, such as mining, and from its various uses in industry, in animal feed, as a wood preservative, and as a pesticide. In drinking-water supplies, arsenic poses a problem because it is toxic at low levels and is a known carcinogen. In 2001, the USEPA lowered the MCL for arsenic in public-water supplies to 10 micrograms per liter (µg/L) from 50 µg/L.
Dangerously high levels of arsenic have been found in drinking water wells in more than 25 states in the United States, potentally exposing 2.1 million people to drinking water high in arsenic. Possibly the worst case ever of arsenic poisoning occurred in Bangladesh, where over 100 million people were poisoned by arsenic in groundwater supplies.
Figure 2: Global map of the probability of geogenic arsenic contamination in groundwater for reducing and high-pH/oxidizing aquifer conditions. From Amini et all. (2008). Statistical modeling of global geogenic arsenic contamination in groundwater. Environmental science & technology, 42(10), pp.3669-3675.
Figure 3: Map showing estimates of how many private domestic well users in each county may be drinking water with levels of arsenic of possible concern for human health.(µg/L, micrograms per liter)
Figure 4: Spatial distribution of elevated (>MCL) arsenic concentrations in groundwater samples from water wells in Michigan, United States (where MCL=maximum contaminant level enforced by the U.S. Environmental Protection Agency). Approximately 28% of the samples in the statewide analysis have concentrations exceed the MCL of 0.01 mg/L.
Figure 5: Global distribution of major reported problems of high fluoride concentrations in groundwater. Source: GroundwaterGoverance.org
Mineralized Groundwater
Figure 6: U.S. coastal areas where freshwater and seawater mix unseen below ground, making them vulnerable to ocean and/or drinking water contamination. Dark blue areas are vulnerable to land-to-sea pollution; pink to sea-to-land pollution; light blue to both. Credit: The Ohio State University/NASA-JPL/Caltech.
Figure 7: Interaction of deep, highly-mineralized brines with shallow freshwater in the Lower Peninsula of Michigan, United States. Based on research by Curtis et al. (2019). A Multiscale Assessment and of Shallow Groundwater Salinization in Michigan”, Journal of Groundwater.
Mine Drainage and Construction
Figure 8: Global distribution of documented problems with arsenic in groundwater (>50 µg/L) related to mining and geothermal activity. From: British Geological Survey https://www.bgs.ac.uk/arsenic/