Hydrology
Lecture 15 Groundwater Quality and Contamination
Reading assignment: Chapter 11
Water Chemistry
Groundwater is never pure, rather, it contains small amounts of dissolved gases and
solids.
Chemical analysis of an aqueous solution yields the amount of solute in a specified
amount of water. There are several ways this is reported:
Weight per Weight units (dimensionless ratios): ppm, ppb, ppt (parts per million,
billion, trillion). These are no longer commonly used in water analysis.
Weight per Volume units: milligrams per liter (mg/L) and micrograms per liter (ug/L).
These are more commonly used.
1 liter of pure water = 1X106 milligrams H2O at 3.89°C therefore 1 ppm = 1 mg/L and 1
ppb = 1 ug/L
Note: density and weight of water will change with temperature and dissolved mineral
matter, but not enough to worry about within the range of normal groundwater
conditions.
Molarity (M) is the number of moles of solute dissolved in one liter of solution
(moles/L). The units, therefore are moles per liter, specifically it's moles of solute per
liter of solution.
One Mole of a chemical species is its formula weight in grams, calculated by adding up
the atomic weights of all the atoms in that species. E.g. NaCl = Na (at.wt. 22.991) + Cl
(at.wt. 35.457) = 58.448 gm.
Thus, a 1 molar solution of NaCl would be 58.5 grams of NaCl dissolved in 1 Liter of
water.
Molality (m) = moles solute / kg of solution
Molarity: 1 mole per liter of solvent = 1 molar (M) solution
Molality: 1 mole per 1000 grams of solvent = 1 molal (m) solution
For water solutions, 1 kg of water has a volume close to that of 1 liter, so molality and
molarity are similar in dilute aqueous solutions – up to .01 molal.
Thus for dilute solutions, you can calculate Molality as:
Molal = mg/L X 10-3 / formula wt in grams
Sample Calculations:
1. What is the molarity of a solution made by dissolving 2.5 g of NaCl in enough water to
make 125 ml of solution?
molarity = moles of solute / liters of solution
2.5 g NaCl x (1 mole NaCl / 58.5 g NaCl) = 0.0427 mole
molarity = 0.0427 mole NaCl / 0.125 L = 0.34 M NaCl
2. How many moles of salt are contained in 300 mL of a 0.40 M NaCl solution?
0.40 moles NaCl / .3 L = 0.12 moles NaCl
3. Example molality calculation:
34 mg/L NaCl = ? molality
Molality = mg/L X 10-3 / formula wt in grams
Formula wt of NaCl = 58.5 gram/mole
34 mg/L x 10-3 / 58.5 = .00058 molal
Equivalent Weight
Equivalent Weight units incorporate the chemical behavior of a solute. The equivalent
weight or combining weight of a dissolved ionic chemical species is the formula weight
divided by the electrical charge.
Example: A divalent ion (Ca+2) combines with Cl- (a monovalent ion) to form calcium
chloride (CaCl2). The combining ratio of calcium to chlorine is one-to-two, meaning it
takes half a mole of Ca+2 to combine with one mole (or 1 eq) of Cl-. So, half a mole of
calcium equals 1 equivalent.
Another way to look at it is to say one equivalent is the amount of ion required to cancel
out the electrical charge of an oppositely charged monovalent ion. You can also say that
the valence charge of the ion is the number of equivalents there are in one mole of that
ion. For example, nitrogen (N-3) is a trivalent ion; therefore one mole of nitrogen equals
3 equivalents. Using the calcium example from above, we know that one mole of calcium
equals 2 equivalents.
The amount of a given substance in equivalents is equal to the amount of the substance in
moles multiplied by the valence of the substance.
The equivalent weight is the mass of one equivalent of a substance.
Example:
1 mol H2SO4 = 2 equivalents H2SO4
There are "TWO" H+ released when the acid dissociates
The acid releases two H+ ions and one SO42- ion.
Thus 1 equivalent weight of sulfuric acid is 1/2 the mass of a mol.
1 mol H2SO4= 98 grams H2SO4; 1 equivalent H2SO4= 49 grams H2SO4
In practice, the amount of a substance in equivalents often has a very small magnitude, so
it is frequently described in terms of milliequivalents (mEq or meq) -- the prefix milli
denoting that the measure is divided by 1000. Very often, the measure is used in terms of
milliequivalents of solute per liter of solvent (mEq/L).
Example:
1 mol H3PO4 = 3 equivalents H3PO4
The acid releases three H+ and one phosphate PO42- ion.
The equivalent weight of phosphoric acid is 1/3 the mass of a mol.
1 mol H3PO4= 98 grams H3PO4; 1 equivalent H3PO4= 32 grams H3PO4
The relation between equivalents and milliequivalents:
1 equivalent H3PO4= 1000 milliequivalent H3PO4
1 equivalent mol H3PO4= 32 grams H3PO4;
1 milliequivalent mol H3PO4= 32 milligrams H3PO4;
Dividing the concentration in mg / L by the equivalent weight of the ion (one equivalent)
yields a concentration in milliequivalents per L (meq/L). Non ionic species such as silica
cannot be expressed in meq/L.
Example:
Ca2+ = 40.08 gm/mole = 2 equivalents
1 equivalent = 20.04 gm
134 mg/L of Ca2+ = 134 mg/L / 20.04 gm = 6.69 meq/L
Equivalents have advantage over moles in quantitative analysis of reactions. The best
feature of using equivalents is that there is no need to study much about the nature of
reaction, i.e. no need to analyse and balance chemical equations. Equivalents of reactants
react in equal numbers to yield the equal equivalents of products.
Groundwater Quality
Any substances introduced into groundwater by human activities are considered
contamination. Natural contamination can also occur if undesirable elements are
introduced into groundwater by natural processes.
Contaminant levels that exceed acceptable limits result in pollution.
The total amount of contaminants in water is often expressed as TDS (total dissolved
solids). This is the total weight of material, in milligrams per liter, that remains if a
sample of water is completely evaporated.
Generally, solids exist in solution as charged ions, either + cations or – anions. Some
solids, such as silica, can exist in solution in a non-ionic state.
The total dissolved solids can be subdivided into three categories based on the relative
amounts of individual contaminants.
Major constituents: these constitute the bulk of the dissolved solids.
Cations: Calcium, magnesium, sodium, and potassium
Anions: Chloride, sulfate, carbonate, and bicarbonate
Non-ionic: Silica
Minor constituents: important dissolved solids present in lesser amounts
Iron, mangenese, flouride, nitrate, strontium, and boron.
Trace elements: these dissolved solids are present in very small amounts. Nevertheless,
many are harmful in trace quantities and their small quantities do not diminish their
import effect on groundwater quality.
Arsenic, lead, chromium, cadmium.
Additionally, gases may be dissolved in groundwater
Microbiological contaminants: such as protoctistans, bacteria, fungi, and viruses can
also be present as contaminants.
Organic substances: The number of man-made organic substances that has been
identified in drinking water supplies was over 1200 as of 1976. Considering that about
300 to 500 new organic compounds are brought into commercial production each year,
this number is undoubtably much higher today.
Many of these organic substances are prevented from reaching the saturated subsurface
through a variety of mechanisms such as volitization, chemical precipitaion, biological
degredation, chemical degredation, and absorbtion.
Furthermore, substances that are not soluble in water tend not to travel more than a few
hundred feet from the source of the contamination. However, most compounds have
some degree of solubility so that small amounts do get into the groundwater.
Unfortunately, many organic substances with low solubilities have even lower
concentrations at which they are harmful to health.
Organic compounds that do not easily degrade and are left remaining as potential
contaminants are called refractory compounds.
The most chronic contamination problems come from those compounds that are soluble,
non-volatile, and refractory.
Water hardness is an important groundwater parameter related to TDS. Hardness is
defined as the content of metallic ions (primarily. Ca+, Mg+) that react with sodium
soaps to form ‘soap curd’ or that react with negative ions to form a scaly crust on pipe
surfaces.
Hardness is a nuisance because it prevents soaps from cleaning until all metallic ions in
the water are used up - much soap is wasted. It also tends to clog pipes in plumbing and
heating systems.
The Environmental Protection Agency has established Recommended Concentration
Limits for many of the major and minor TDS constituents. These limits mark the
concentrations at which the quality of the groundwater for human use becomes adversely
affected.
These are not neccessarily the concentrations at which the substances are dangerous. For
example, the RCL for iron is .3 mg/L. At this concentration iron is actually beneficial as
a dietary nutrient, but it causes discoloration of surfaces and laundry stains, and is
considered unacceptable for tap use.
TDS constituents that involve health risks are assigned Maximum Permissible
Concentrations.
Radioactive TDS constituents are regulated based on a Maximum Permissible Activity,
usually measured in picoCuries per liter.
The Safe Drinking Water Act (Public Law 93-523) passed by Congress in 1974 set
standards for safe drinking water to protect public health. Amendments passed in 1986
substantially enlarged the list of regulated substances.
Under the Safe Drinking Water Act, two levels of contamination are specified.
Maximum contaminant level goals (MCLGs) are non-enforceable health goals – levels
of contamination that would ideally not be exceeded.
Maximum contaminant levels (MCLs) are enforceable standards that are set as close to
the MCLGs as is feasible on the basis of water treatment technologies and costs.
Additionally, suggested maximum contaminant levels (SMCLs) are set at
concentrations at which contaminants begin to effect the taste and aesthetics of drinking
water.
The specific values of the MCLGs and MCLs are continually being updated by the EPA.
Updated values are published in the Federal Register.
Amendments made in 1986 to the Safe Drinking Water Act also established a general
aquifer classification and regulated the injection of contaminated water back into
aquifers.
The Gonzales Amendment authorized the division of aquifers into three types:
1. Class 1: Special Ground Waters – those that are highly succeptible to contamination
and that are an irreplacable source of drinking water or water supplied to an
ecologically sensitive area.
2. Class 2: Current and potential sources of drinking water
3. Class 3: Groundwaters not considered potential sources of drinking water
Class 1 aquifers are the most heavily regulated, followed by class 2 and class 3.
Nassau County, Long Island (information from Department of Health Ground Water
and Public Water Supply Facts for Nassau County, 1998)
1. In 1998, Nassau County had 54 public water systems withdrawing water from 398
public supply wells - 81% Magothy, 10% Lloyd, 7% Upper Glacial.
2. No source water monitoring violations or MCL violations for organic and inorganic
chemical contaminants in 1998.
3. In 1998, 81 (20%) of public supply wells treated water for VOC contamination using
Packed Tower Aeration or Granular Activated Charcoal.
4. 40% of public supply wells tested in 1998 had detections of VOC - most frequently
tetrachloroethylene (22%), trichloroethylene (20%), and 1,1,1 trichloroethane (16%).
5. MTBE was detected in 19 (6%) of public supply wells tested in 1998.
6. 99% of public supply wells exhibited no pesticide contamination. Ethylene
dibromide was detected in one well on the North Shore at levels exceeding the MCL.
This well is using Granular Activated Charcoal to treat for this contaminant.