Goals

• Describe the two-way relationship between water resources and human society
• Explain the distribution and dynamics of water at the surface and in the subsurface of the Earth
• Synthesize data and information from multiple reliable sources
• Interpret graphical representations of scientific data
• Identify strategies and best practices to decrease water stress and increase water quality
• Thoughtfully evaluate information and policy statements regarding water resources
• Communicate scientific information in terms that can be understood by the general public

Learning Objectives

In completing this module, you will:

• Calculate the concentration of contaminant in a reservoir
• Apply government drinking water regulatory standards to identify contaminant levels that might be harmful to human health
• Analyze concentration vs. time data for various dissolved components of river water and ground water
• Infer the processes responsible for seasonal trends in compounds of natural and human origin
• Propose and evaluate methods for mitigating human impacts on water quality
• Evaluate the tradeoff between agricultural productivity and water quality as a result of fertilizer usage and runoff

Elements and molecules have solubilities (the amount that can be dissolved in water before the water is saturated with that element and no more can be held in solution) that depend on their individual properties and styles of bonding to other elements. For example, common table salt (NaCl), when added to water, will dissociate into individual charged ions of Na+ and Cl-. These are held apart from one another "in solution" because they are surrounded and isolated by polar water molecules (Unique Properties of Water section).

Distilled water or "pure" water typically has near-zero concentrations of other components. If concentrations of dissolved elements or compounds are present, they are usually expressed in terms of mass (weight) of the component/unit volume of water, mass element/mass water, or moles element/mass or volume of water.

Typically, the volume of water referenced is a liter (1000 grams--1 kg by mass), and the elemental or component mass is in grams (or milligrams, mg). Milligrams/Liter (mg/L; 0.001g/1000g) or milligrams/kg (mg/kg) is the same as parts per million (ppm) as concentration. You will often see a concentration of a dissolved species in water expressed in either mg/L or ppm.

Molar concentrations, commonly used by chemists, are expressed as a decimal fraction of the mass of Avogadro's Number (a mole) of atoms (6.022 x 10²³) of a given element or elements in a compound, equivalent to atomic or molecular mass. For example, a mole of carbon (¹²C) has a mass of 12 grams, and a mole of carbon dioxide (CO₂) has a mass of 44 grams (¹²C, ¹⁶O, ¹⁶O). So, if a liter water sample contains 0.044g of carbon dioxide (44 ppm), the carbon dioxide concentration would be 0.01 mole/kg.

Who Regulates Drinking Water Quality in the U.S.?

Public drinking water quality is regulated by the U.S. Environmental Protection Agency (EPA) by provisions of the Safe Drinking Water Act (SDWA), although individual states can apply and enforce their own standards if more stringent than those set by the EPA. The SDWA was originally passed by the U.S. Congress in 1974, and has been amended twice (1986, 1996) and now provides standards for drinking-water sources, treatment, and quality at the tap, as well as the disposal of wastewater underground. Private wells pumping groundwater that serve fewer than 25 people are not regulated. They should be tested regularly, however.

It is estimated that there are over 160,000 public drinking-water systems that fall under the aegis of the EPA standards. These standards are health-based and attempt to establish maximum levels (MCL—Maximum Contaminant Level) for possible contaminants that are below those that are thought to cause health problems (you can see specific contaminants and MCLs at EPA: Drinking Water Contaminants - Standards and Regulations). Of course, there are many contaminants for which there are insufficient data to establish stringent limits.

Over the past decade, bottled water, usually sealed in "plastic" containers has become quite popular worldwide. Accoring to bottledwater.org, in the U.S., over 15.9 billion gallons of bottled water were sold in 2022, revenues were more than $46 billion, assuming an average price of $1 per liter. Although convenience is certainly a factor, the perception has been that such water must be safe to drink—perhaps more safe than tap water—also drives bottled water sales. In the U.S., bottled water is actually regulated by the U.S. Food and Drug Administration (FDA), not the EPA. The FDA regulates bottled water as a food (requiring compliance with the Federal Food, Drug and Cosmetic Act) and does not require certified lab testing or violation reporting, even though the FDA does inspect bottling plants and ensures that suitable source waters are used. The FDA also has generally adopted limits for contaminants established by the EPA. Nonetheless, the FDA does not require bottled water companies to disclose to consumers the source of the water, treatment processes, or contaminants it contains, whereas the EPA requires public water systems to report results of their testing annually.

Natural waters have a broad range of total dissolved solids (TDS). Some fresh mountain streams might have TDS concentrations less than 250mg/kg. Seawater, on average, has TDS concentrations of nearly 35g/kg. Extreme TDS values are found in highly evaporated lake or isolated seawater basins and in the deep subsurface (so-called "formation waters"), with TDS of nearly 350g/kg (35% salt solution!). We will focus here briefly on the compositions of potential drinking water sources (rivers and lakes) and the origins of the dissolved species.

Flowing water, whether in aquifers or streams, interacts with rocks and soils and slowly dissolves some of their chemical constituents. The pH (hydrogen ion activity) of the water determines the rate of dissolution and solubility of many chemical species. However, we will not discuss chemical processes in any detail here. Some chemical substances, particularly redox-sensitive trace metals (e.g. Fe, Mn, Pb, As and others), are more soluble when natural waters are depleted in dissolved oxygen (see the section called Contaminant Example 2 below). Most chemical species in natural waters have both natural and pollutant sources of many types (Table 1).

Natural waters also contained dissolved gasses. For example, carbon dioxide from the atmosphere is dissolved in water, and, through a series of chemical reactions, contributes to the total dissolved carbon in waters—primarily bicarbonate (HCO₃^2-). Gas solubility is inversely proportional to temperature and TDS. For example, dissolved oxygen solubility is shown as a function of temperature and salinity in Figure 1. Note that the amount of oxygen that can be held in fresh water decreases nearly 50% from near freezing temperature to 35°C. These are maximum concentrations, but natural waters can have lower dissolved oxygen concentrations as the result of biological activity such as the metabolism of water inhabitants, including bacteria. Photosynthesis of algae and aqueous plants can add oxygen to the water in which these primary producers grow. However, the breakdown of organic material by bacteria consumes dissolved oxygen. Thus, in waters below the surface wind-mixed layer (usually tens of meters or more) or in stably stratified lakes or bays, for which rates of oxygen replenishment to deeper depths are slow, deficiencies in dissolved oxygen can develop, with anoxia (total depletion of dissolved oxygen) at the extreme. Excess nutrient supply can have the same impact on a water body (eutrophication: see Module 1 and Contaminant Example 2: "Dead Zones" and Excess Nutrient Runoff) with deleterious effects on the aquatic biota.

Figure1. Dissolved oxygen (ml/L, or ppm) solubility (maximum expected concentration) in waters of different salinity (parts per thousand). Seawater typically has a salinity of 35 parts per thousand, whereas fresh water is near 0 parts per thousand.

Source: Michael Arthur, Penn State

There are, of course, many possible contaminants in drinking water supplies—in part natural, but also induced by human activities. There are three main groups of contaminants with relation to anticipated health effects:

1. Some contaminants produce no health effects until a threshold concentration is exceeded. Nitrate (NO₃) is an example of this; OK at 50 mg/liter (50 ppm), but at higher levels, it produces methemoglobinemia (e.g., "blue baby" syndrome).
2. The second group of contaminants has no apparent threshold for health effects. These include genotoxic substances. These include some natural and synthetic organic compounds, micro-organic compounds, some pesticides, and arsenic (see below) for example.
3. A third group consists of elements essential to the human diet: fluoride, iodine, and selenium are good examples—their absence in the diet causes problems, but an excess of intake can create problems.

Arsenic is a good example of both natural and human-induced contamination, and it is important as well because of its toxicity at higher concentrations (as are lead and fluoride). In recognition of the potential toxicity of arsenic (As), the US EPA lowered the MCL in drinking water from 50 ppb to 10 ppb in 2001. For example, check out this short video on possible health effects of arsenic and the need to have private wells tested.

Video: In Small Doses: Arsenic (10:01)

The health effects of arsenic.

In the western US, groundwater As levels are particularly high (see Fig. 2) because of the types of bedrock the groundwater moves through. The high concentrations in Maine are due to more alkaline (high pH), low dissolved oxygen groundwater that leads to high solubility of arsenic in shallow aquifers of glacial origin. Contamination of aquifers can also occur from agricultural runoff, runoff from arsenic-bearing wood preservatives, improper disposal of chemicals containing As, and/or mining activities. See this article in The New York Times, The Arsenic in Our Drinking Water, for a summary of possible health effects in the U.S. and Bangladesh.

Figure 2. A map of the US showing median arsenic concentrations (μg/L or ppb) in groundwater used to supply drinking water.

Source: U.S. Geological Survey

Global maps of the probability of arsenic concentrations in groundwater greater than 10 ppb can be viewed at Global fluoride and arsenic contamination of water mapped and is based on research by M Amini et al (Environ. Sci. Technol., 2008, DOI:10.1021/es702859e. A more generalized map of risk for As in drinking water can be seen in Figure 3.

Figure 3. Modeled risk of As in drinking water at significant levels from Schwarzenbach et al., 2010.

Source: United Nations Environment Program (UNEP)

A major issue in pollution of surface waters is the role that excess nutrient flows from polluted waterways into lakes, bays, and coastal zones play in creating excess biologic production in surface waters and dissolved oxygen at depth. In most cases, this nutrient-rich runoff results from agricultural operations, including the application of fertilizer to crops. Of course, such issues have already been briefly highlighted for the Chesapeake Bay in Module 1, but such so-called "Dead Zones" are globally widespread. It is, perhaps, easier to understand impacts on more restricted bodies of water (lakes, bays) with high fluxes of water from nutrient-laden rivers (such as the Chesapeake Bay setting). But, such issues also plague some coastal zones characterized by high river discharges. For example, the Gulf Coast "dead zone" has been recognized for over a decade and is attributed to high rates of nitrogen (and phosphorus) discharge through the Mississippi River system. Watch the following video from NOAA that provides a dead zone 'forecast' for 2019 and explains in general how dead zones form in the Gulf of Mexico and their impacts on the region.

Video: Happening Now: Dead Zone in the Gulf 2019 (1:59)

Dead Zone 2019 forecast and explanation

During summer, 2014, this area of hypoxia (less than 2 ppm dissolved oxygen in the water column near the bottom on the shelf) along the Louisiana and Texas coast was just over 13,000 km2 (>5000 mi2), somewhat smaller than that in 2021. Figure 7 illustrates the extent and severity of oxygen deficiencies during mid-summer, 2021. Coastal currents flowing westward mix and transport nutrients flowing from the Atchafalaya and Mississippi Rivers into the ocean.

Figure 7. Contours of dissolved oxygen near the bottom on the Louisiana and Florida shelves, July 25-31, 2021. Note deep red areas outlined in black indicating widespread hypoxia.

Source: NOAA

But how do high nutrient fluxes promote oxygen deficiency in coastal regions? The availability of nutrients in shallow sunlit waters near the coast allows prolific blooms of marine plankton (primary photosynthesis) which produces large amounts of organic matter. Nutrients can be a good thing and can benefit the entire food chain unless the fluxes of N and P reach an extreme termed "eutrophic" conditions. As the organic matter sinks to the bottom, it is a food source for consumer organisms (both in the water column and on the bottom), including bacteria. Shrimp, bivalve, and fish catches can increase to a point. In the extreme, the metabolism of fish, bivalves, bacteria and other critters consumes available dissolved oxygen in the water column faster than it can be replenished by mixing from above or laterally by currents. Also, because the coastal waters are warming during summer, they can hold less dissolved oxygen initially. As long as high nutrient fluxes continue the hypoxia expands and the organisms that depend on oxygen to survive either flee if they can swim, or die if they are more sedentary.

Observations of nearly 40 years indicate that the extent of hypoxia can wax and wane from year to year. In 2021, the Mississippi River saw increased discharge and nutrient runoff prior to the hypoxia event. In 2023, Louisiana coastal hypoxia was much less extensive and intense (Fig. 8, contrast with Fig. 7).

Figure 8. Contours of the dissolved oxygen near the bottom on the Louisiana and Florida shelves, July 23-28, 2023 (top). Bar graph of area in the Gulf of Mexico with hypoxia by year (bottom).

Source: NOAA

Previous research established a connection between runoff from agricultural operations in the mid-continent region into the Mississippi River drainage and development of hypoxia. Wet years (Fig. 9 corresponds to higher flow rates for the Mississippi River and greater delivery of dissolved nitrogen to the coastal region. Note that 1987-89 were years of low nitrate flux (Fig. 9), which correspond to low area of Gulf of Mexico hypoxia.

Figure 9. Nitrate flux from the Mississippi River scaled to Mississippi River flow rates (right y-axis in millions of cubic meters/y) to the Gulf of Mexico. Overall, there is a correlation between the two factors, particularly after about 1970. This study found a strong correlation between nitrate flux to the Gulf of Mexico, annual discharge to the Gulf, and fertilizer application over the entire drainage basin during the previous two years (r2=0.89).

Source: From Goolsby and Battaglin, 2000, USGS Fact Sheet 135-00

Figure 10. An estimated 5.5 million metric tons of nitrogen fertilizer were applied to croplands in the Mississippi River Basin during 1991.

Source: From Goolsby and Pereira, 1995; USGS Circular 1133

It is also clear from Figure 10 that very high rates of fertilizer application characterize the Mississippi River Basin. Think back to the section called Contaminant Example: Arsenic in Groundwater when you examined nitrate concentration variation in Iowa streams at present. It should be apparent that fertilizer applications and runoff are the main culprits in hypoxia in the Gulf of Mexico.