The Chemistry of Natural Waters

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The Chemistry of Natural Waters

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).

Table 1: Most common inorganic substances found in natural waters on land and their dominant sources (Berner and Berner, 1996)
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

Key for Table Above

  1. wind-blown sea salt
  2. soil dust
  3. biogenic aerosols
  4. lightning and N2 in atmosphere
  5. biological decay
  6. volcanic activity
  7. carbon dioxide in air
  8. biomass burning
  9. cement manufacture
  10. fuel combustion
  11. automobile emissions
  12. land clearing
  13. gas reactions
  14. fertilizers
  15. industrial chemicals

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.

Dissolved O2 starts between 7-11 ml/L least salinity has the greatest amount. All decrease as temperature increases 2 between 4-5 ml/L.
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

Activate Your Learning

Go to: the USGS Water Quality Watch website 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.

Questions

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?).


Click for answer.

ANSWER: Oxygen concentration varies seasonally with temperature. Lower concentrations occur during times of warmer temperature when oxygen solubility is lower and demand by respiration of organisms is higher. In the winter, solubility is higher and demand lower so concentrations peak.

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.


Click for answer.

ANSWER: In 2014, Illinois, Iowa, and Missouri had the highest nitrate levels. Most likely this is due to agricultural runoff (fertilizers) in these dominantly agricultural states.

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?


Click for answer.

ANSWER: The answer vary will depend on timing. Generally, the nitrate levels in the Boone River are higher than drinking water standards. Nitrate concentrations vary on a weekly to monthly basis as the result of runoff from croplands and/or stockyards/CAFOs. If it has rained in the past week, you should be able to test whether higher concentrations occur as the result of precipitation events with runoff contributing more nitrate to streams or whether precipitation and runoff events dilute stream nitrate concentrations.

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?).


Click for answer.

ANSWER: Highest specific conductance is typically found in coastal regions where fresh water sources mix with varying proportions of salty seawater in estuaries due to tide and wind mixing. Highest TDS values exceed 25,000 ppm (about 70% seawater).

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?

Click for answer.

ANSWER: Most of these streams undergo considerable evaporation during the dry season, causing TDS to increase. The minimum specific conductance value of 2400 μS/cm is equivalent to about 1536 ppm TDS, which is about 3 times the allowable drinking water standard. Most likely, people in this area of North Dakota draw on deeper groundwater recharged by streams originating in the mountains to the west.

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)?

Click for answer.

ANSWER: Interestingly, the Pecos River drains basins that contain "evaporite" strata (former marine environments produced halite—NaCl—by extreme evaporation). This type of rock is highly soluble in fresh water. It is also possible that a number of dams on the Pecos contribute to increases in TDS through long-term evaporation in this arid environment.