Water is the "Universal Solvent." Virtually every element on the periodic table and many organic substances (molecules) are soluble (can be dissolved) to some degree in water. Many substances occur "naturally" in water—that is they are dissolved into water as it flows over rock surfaces or through aquifers in the subsurface or as it mixes with other waters. Some substances are "pollutants," having been added as the result of certain human activities, intentionally or unintentionally, including wastewater (untreated) disposal, drainage of acidic waters from abandoned mines, drainage from agricultural operations (e.g. manure, herbicides, pesticides), etc. "Water quality" implies an assessment of the degree of contamination of a water source by direct measurement of its dissolved components. Not all dissolved components in water are harmful to human health, but this depends, in part, on their concentration. In this module, we will explore some of the science and issues with respect to drinking water quality, a bit about the chemistry of natural waters, and the regulations that help ensure a satisfactory drinking water supply for the U.S. populace. In addition, we will outline some water quality issues that affect other parts of the globe.
In completing this module, you will:
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.
Total dissolved solid concentrations (TDS; concentrations of all dissolved inorganic species) for water samples can be fairly accurately measured by
1. The Na concentration in a water sample is 10 ppm. What is the concentration expressed in g/kg?
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2. The mass of a mole of sodium (Na) is about 23 g. A water sample has a dissolved sodium concentration of 0.046 g/kg. What is the Na concentration expressed as moles/L?
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3. Read this article about a contaminated water supply [1]. After mixing in with the 38 million gallons of fresh water already in the reservoir, how many parts per million “contaminant” are there (in other words, what is the proportion of the contaminated water to the total volume)? Assume that the input of “contaminant” is 8 oz.
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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 [2]). 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. In the U.S., over 10.1 billion gallons of bottled water were sold in 2013, (according to the International Bottled Water Association [3]), revenues were more than $40 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.
Public water systems are required to analyze their water monthly for a number of possible contaminants and to meet standards set by the EPA. Download the most recent (4-page pdf) Report of the State College Borough Water Authority [4].
Read and then answer the question in the space provided. Click the "Click for answer" button to reveal the correct answer.
1. What is the difference between an AL (Action Level), MCL (Maximum Contaminant Level), and an MCLG (Maximum Contaminant Goal)?
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Maximum Contaminant Level (MCL) – The highest level of a contaminant that is allowed in drinking water. The MCL is enforceable by public health agencies.
Maximum Contaminant Level Goal (MCLG) – A level of a contaminant in drinking water that is thought to be a risk to human health, but not a certainty. Usually, MCLGs are not enforced by public health agencies.
Action Level (LA) – a level of exposure considered hazardous in water, or exposure to a harmful substance that requires remediation.
2. Were any dissolved constituents near the MCL? If so, which ones? What is the most likely source of contaminants for the State College water source?
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3. Look up the drinking water report for your hometown. Answer question 2 for your hometown. If you grew up in a rural community and used well water, was your water analyzed or treated? How?
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4. Do a bit of research online and briefly outline at least one significant difference between EPA drinking water regulations and FDA bottled water regulations (one not already outlined above).
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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).
Ion (molecule) | Natural Source | Pollutant Source |
---|---|---|
Sodium (Na+) | 1, 2 | 8 |
Magnesium (Mg+) | 1, 2 | 8 |
Potassium (K+) | 1, 2, 3 | 8, 14 |
Calcium (Ca+) | 1, 2 | 8, 9, 10 |
Hydrogen (H+) | 13 | 10 |
Chloride (Cl-) | 1 | 15 |
Sulfate (SO42-) | 1, 2, 5, 6 | 8, 10 |
Nitrate (NO32-) | 4, 5 | 8, 10, 11, 14 |
Ammonium (NH4+) | 5 | 14, 5 |
Phosphate (PO43-) | 2, 3, 5 | 8, 14 |
Bicarbonate (HCO3-) | 7 | 7 (5, 8, 9, 10, 11, 12) |
SiO2, Al, Fe | 2 | 12 |
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 (HCO32-). 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.
Go to: the USGS Water Quality Watch website [5] and examine the various maps showing aspects of surface water quality for U.S. monitoring stations (Temperature, conductivity (salinity in ppm), pH, dissolved oxygen (D.O.), turbidity, nitrate (ppm), discharge).
Once you are ready, answer the questions in the spaces provided below. Click the "Click for answer" button to check your answer.
1. Animate the map for dissolved oxygen in surface waters for the past year (a clickable link). Watch the eastern half of the U.S. carefully and describe the trends in DO that you observe. Why does DO in this region vary the way it does (e.g., what is the main control and how does it work?).
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2a. Click on the map for nitrate. Notice that there are many fewer stations with such data because it is more difficult to routinely measure nitrate concentrations. The available stations are probably mostly monitored because the waterways are in some way impaired.
What are the states (three) with the highest nitrate concentrations? Speculate as to the possible causes(s) of high nitrate in waterways in these states.
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2b.Click on the State of Iowa. Then click on one of the monitoring stations (try Boone River near Webster, IA. What is the current nitrate concentration? Is this above or below drinking water standards? Click on "nitrate graph." How has nitrate varied over the past week? Why would nitrate concentration vary? Suggest a way to back up your answer with available data for that site; does it work?
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3a. Click on the map for specific conductance (μS/cm or microSiemens/cm, a measurement of TDS concentration if properly calibrated: use 1000 μS/cm = 640 ppm as TDS, and the scaling is roughly linear, e.g., 103 μS/cm = 6.4 x103 ppm TDS).
Where are surface waters with the highest specific conductance? Why are they high? What is the approximate TDS value for the highest stations (above what value?).
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3b. Why are there a number of streams in the continental interior that have values above 2400 μS/cm? What is this minimum value in TDS? Check out North Dakota, for example. Does a stream with above 2400 μS/cm specific conductance meet drinking water standards? If not, where do you think the drinking water in that area comes from?
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3c. Many of the streams that have relatively high specific conductance observed in question 3b, vary over the year (animate the map and revise your answer to 3b if you see a pattern). However, the specific conductance of the Pecos River in Texas does not vary much (it stands out in southwest Texas) and is quite high. Provide possible reasons why (hint: think about types of rocks that might be in its drainage)?
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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:
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.
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 [6], for a summary of possible health effects in the U.S. and Bangladesh.
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 [8] 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.
An example of a very serious arsenic problem in groundwater is that of Bangladesh. The issue there is related to high rates of groundwater extraction through shallow wells in conjunction with shallow groundwater pollution that caused anoxia at shallow depth (see Fig. 5). The arsenic is associated with the anoxic zone which has been tapped by hundreds of thousands of shallow "tube wells" since the 1980s (Fig. 4), an innovation that saved millions from potential disease, including death by cholera, associated with getting their water from shallow pits. Ultimately, the new deeper water source began poisoning them with arsenic (Bhattacharjee, et al., 2007, Science 315, p.1659) liberated from iron oxides that were "reduced" under anoxic conditions, thereby liberating adsorbed As into dissolved form in the groundwater.
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 [11] 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.
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 2013. Figure 6 illustrates the extent and severity of oxygen deficiencies during mid-summer, 2013. Coastal currents flowing westward mix and transport nutrients flowing from the Atchafalaya and Mississippi Rivers into the ocean.
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 over a number of years indicate that the extent of hypoxia can wax and wane from year to year. In 2012, Louisiana coastal hypoxia was much less extensive and less intense (Fig. 8, contrast with Fig. 7). A severe drought occurred in 2012 in the mid-continent U.S. The flow of the Mississippi River system was much reduced, and nutrient fluxes decreased commensurately.
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.
Links
[1] http://www.bbc.com/news/world-us-canada-27069611
[2] http://water.epa.gov/drink/contaminants/index.cfm for specific contaminants and MCLs
[3] http://www.bottledwater.org/economics/bottled-water-market
[4] https://www.scbwa.org/water-quality-reports
[5] http://waterwatch.usgs.gov/wqwatch/
[6] https://well.blogs.nytimes.com/2013/09/20/the-arsenic-in-our-drinking-water/
[7] https://www.usgs.gov/mission-areas/water-resources/science/arsenic-and-drinking-water
[8] https://www.chemistryworld.com/news/global-fluoride-and-arsenic-contamination-of-water-mapped/1015707.article
[9] http://geodev.grid.unep.ch/extras/geg_slider.php#
[10] https://en.wikipedia.org/wiki/Tube_well
[11] https://oceantoday.noaa.gov/happenowdeadzone/
[12] http://www.ncddc.noaa.gov/hypoxia/products/2000/