8.4: Natural Effects on Water Quality

For water to be useful, it must be of suitable quality. There are a range of factors that affect the quality of potential water resources.

Fresh water on the surface (in streams or lakes) has limited opportunity to react with the surrounding materials, and so its natural chemical composition (excluding human-sourced contamination) is not typically a factor in terms of its quality. Groundwater, on the other hand, is in close contact with the rock or sediment through which it is moving and there is plenty of opportunity for the water chemistry to be changed by interaction with minerals. Several factors affect how much the interaction of the water and mineral materials changes the composition of the water. The amount of time the groundwater is in contact with the materials plays a large role. The longer the water sits there, the more the materials can dissolve and contaminate the water. The depth of the groundwater is typically related to how old the water is as older groundwater is typically found at deeper depths. Consequently, deeper water tends to be more contaminated. And finally, the materials the water sits in plays a role. Some geologic materials dissolve quite easily, while others do not.

When geologists describe water quality, different terms may be used. Total Dissolved Solids (TDS) is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid. It is often measured in parts per million (ppm) or milligrams per liter (mg/L). Concentrations of some of the common naturally occurring chemicals in water are summarized in this section. Tolerances are often dependent upon use where water for farming, water used by households, and drinking water need to be increasingly pure in that order.

Concentrations of Major Constituents

Figure \(\PageIndex{1}\) shows the concentrations of the major constituents in groundwater from different parts of a sandstone aquifer on Vancouver Island and in surface waters from the same region. These values could be somewhat representative of aquifers in other locations, as these seven constituents are the dominant components of most groundwaters. Bicarbonate (HCO^₃-), sodium (Na⁺) and chloride (Cl^-) are present at the highest concentrations in these waters—all between 70 and 120 mg/L—followed by sulphate (SO^₄-2) and calcium (Ca⁺²) at around 20 mg/L. If we were to add up all these numbers the total would be close to 350 mg/L, which means that if we evaporated a liter of this water, we would be left with 0.35 g of salts, or a little less than 1/10th of a teaspoon. Water with over 1000 mg/L of dissolved solids is unlikely to be good to drink. Sea water has approximately 35,000 mg/L of dissolved solids (about 7 teaspoons), mostly as sodium and chloride.

Figure \(\PageIndex{1}\): The average concentrations of the major components of groundwater from a sandstone aquifer, and surface water in the Vancouver Island area, BC

Concentrations in surface waters (streams and lakes) from the same areas of Vancouver Island are much lower (20% as high on average), although K and Mg are a little higher in the surface waters than the groundwaters. One liter of this water contains about 70 mg of the salts of these elements.

Concentrations of some of the important minor constituents in water samples from the same area are shown on Figure \(\PageIndex{2}\). The most abundant of these elements is silicon (Si), followed by iron (Fe) and fluoride (F). Note that this graph has a logarithmic scale; the average concentration of Fe is less than 1/10th that of Si, and Al is about 1/100th that of Si. The total contribution of all these elements is equivalent to 6 mg in a liter of water.

Figure \(\PageIndex{2}\): The Average Concentrations of Some of the Minor Components of Groundwater from a Sandstone Aquifer, and Surface Water in the Vancouver Island Area, BC. On average, the surface waters represented here have minor element concentrations that are about 1/5th as high as the groundwaters.

pH

Hydrogen ion concentration of water is expressed in pH units. Most waters have pH within the range 6 to 8, where anything less than 7 is acidic. It is not uncommon for surface waters to have pH levels of less than 6, especially in areas where the water is draining over rock with even low levels of pyrite (FeS₂). A low pH is not necessarily a concern, but because most metals are more soluble at lower pH, acidic water tends to have higher concentrations of heavy metals such as copper and zinc. Acidic water can also be a problem in buildings because it will contribute to corrosion of metal pipes. It’s also quite common for surface water to have a pH well above 7, especially in areas underlain by limestone. Groundwater can also be acidic or basic (pH greater than 7). As discussed below in the section on fluoride, while a high pH is not typically a problem itself, it can be associated with other issues.

Hardness

One of the most common issues with groundwater is hardness, which is a measure of the combined concentrations of calcium and magnesium. Water is considered “hard” if it has hardness above 80 mg/L (Ca plus Mg expressed as CaCO₃ equivalent) and this is common with waters that are derived from sandy aquifers or from limestone aquifers. Hard water inhibits the activity of soaps and detergents, so makes washing of clothes or dishes less effective than it is with soft water. Water softeners are widely used to treat hard water. These are effective in removing most of the Ca and Mg, but do so by replacing them with Na. Excess Na consumption is a health risk to many people, so drinking or cooking with softened water is not recommended.^[1]

Iron

Like most other elements, iron can exist in more than one oxidation state; the two most common are Fe²⁺ (ferrous iron) and Fe³⁺ (ferric iron). Ferrous iron is quite soluble, while ferric iron is virtually insoluble, except at very low pH. While the ground and surface waters shown on Figure \(\PageIndex{2}\) have iron concentrations of around 0.2 mg/L, ferrous iron is present at up to 10 mg/L in some deep groundwater.^[2] When water of this type comes to surface the iron is quickly oxidized to the ferric state, and it precipitates as iron oxide and iron hydroxide minerals. An illustration of the effects this process is provided on Figure \(\PageIndex{3}\):. Rusty stains like this are also common in kitchen and bathroom fixtures in places where the water source is rich in iron.

Figure \(\PageIndex{3}\): Rusty Stains Associated With the Discharge of Iron-Bearing Groundwater from a Sandstone Aquifer on Vancouver Island

Fluoride

Fluoride is beneficial for dental health at low levels, but too much fluoride can lead to discoloration and malformation of teeth, and excessive fluoride over decades can lead to crippling skeletal problems. Almost all surface water and most groundwater has less than 0.5 mg/L (<500 µg/L) fluoride, but some groundwater has over 1.5 mg/L, which is the international maximum acceptable concentration (MAC) for fluoride. Fluoride solubility is low in water that has significant calcium concentrations and relatively low pH.

If groundwater has an elevated fluoride level, it's not likely because the aquifer materials (rock or sediments) have particularly high fluoride levels. Instead, there are water-rock interactions that make fluoride more soluble than it is under normal conditions, allowing fluoride levels to become elevated, even where there is relatively little fluorine in the surrounding material. That process is called base-exchange softening, and it is like what happens inside a water softener. The higher the pH, the more "exchanging" occurs resulting in more flouride in the water.

Arsenic

In most ground and surface waters arsenic (As) concentrations are typically below 5 µg/L, and so are below the international MAC of 10 µg/L, but in some situations arsenic levels can get much higher.

Figure \(\PageIndex{4}\): Areas of Significant Groundwater Arsenic Contamination in Bangladesh

In the floodplain of the Ganges and Brahmaputra Rivers in Bangladesh over 100 million residents extract groundwater from small “tube wells” (wells with diameters of less than about 10 cm). About 8 million such wells were installed in Bangladesh between 1960 and 1990, many with assistance from UNICEF. Prior to that time most rural Bangladeshi’s relied on surface water sources, many of which were contaminated by bacteria and viruses. Gastrointestinal illnesses from these water sources were major health threats. The sediments surrounding groundwater wells are often sufficient to filter out many bacterial and viral contaminants, and thus provision of clean groundwater was celebrated as a major public health success.

However, in the mid-1990s it was discovered that many of the tube wells in Bangladesh have As levels above 50 μg/L ( Figure \(\PageIndex{4}\)), and as many as 20 million Bangladeshi’s are at risk of As poisoning (outcomes include cancer, diabetes, thickening of the skin, liver disease and digestive problems).

Turbidity

Turbidity is a measure of the amount of solid matter—clay and silt plus fine-grained organic matter—suspended in water. Turbid water may look cloudy, although at relatively low turbidity levels—which can still be dangerous—turbidity isn’t detectable without an instrument. Turbidity isn’t an issue because the suspended particles themselves are a health risk, but because they inhibit the effectiveness of disinfection processes. In turbid water biological pathogens attached onto clay mineral grains are effectively protected from chlorine or ozone disinfection, or they can literally be hidden from ultraviolet radiation.

Groundwater is rarely turbid because of its inherent self-filtering by the aquifer, but surface water is often turbid, especially in regions with steep terrain ( Figure \(\PageIndex{5}\)) or following a storm or a debris flow. Most turbidity results from natural processes, but it can be exacerbated by human activities, especially farming, logging and construction.

Figure \(\PageIndex{5}\): Turbid Waters of a Glacier-Sourced Stream in the Canadian Rockies

Although it may not seem like a serious issue, elevated turbidity is one of the most common reasons for the declaration of boil-water advisories. Water suppliers that rely on surface water need to monitor turbidity carefully and continuously and to ensure that their filtration measures are effective. In order to avoid this problem, some water suppliers maintain a surface water supply for most of the year but have backup groundwater wells that they can draw upon when the turbidity of the surface water is too high.

Media Attributions

• Figure \(\PageIndex{1}\): Steven Earle, CC BY 4.0, based on data in Allen, D. and Suchy, M. (2001). Geochemical evolution of groundwater on Saturna Island, British Columbia. Canadian Journal of Earth Sciences, 38(7), 1059-1080. https://doi.org/10.1139/e01-007; and in Earle, S., Krogh, E. (2004) Groundwater geochemistry of Gabriola. Shale: Journal of the Gabriola, Historical & Museum Society, No. 7, 37-44. https://www.researchgate.net/publica...ry_of_Gabriola’s_groundwater
• Figure \(\PageIndex{2}\): Steven Earle, CC BY 4.0, based on data in Earle, S. and Krogh, E. (2004). Groundwater geochemistry of Gabriola. Shale, No. 7, 37-44. https://www.researchgate.net/publica...ry_of_Gabriola’s_groundwater
• Figure \(\PageIndex{3}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{4}\): Groundwater Arsenic Contamination in Bangladesh from Ahmad, S., Khan, M., Haque, M. (2018). Arsenic contamination in groundwater in Bangladesh: implications and challenges for healthcare policy. Risk Management and Healthcare Policy, 11, 251-261. https://doi.org/10.2147/RMHP.S153188. CC BY-NC 3.0)
• Figure \(\PageIndex{5}\): Steven Earle, CC BY 4.0

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1. Health Canada. (2019). Guidelines for Canadian drinking water quality—Summary table. Water and Air Quality Bureau, Healthy Environments and Consumer Safety Branch. https://www.canada.ca/content/dam/hc..._recom-eng.pdf ↵
2. Hem, J. (1985). Study and interpretation of the chemical characteristics of natural water (3rd ed.). U. S. Geological Survey Water Supply Paper 2254. https://pubs.usgs.gov/wsp/wsp2254/pdf/wsp2254a.pdf ↵