Total Dissolved Solids Information

Background

Total dissolved solids (TDS) consists of minerals, organic matter, and nutrients that have dissolved in water - the ions and compounds that you cannot see in the water. Water is known as the universal solvent because of its ability to dissolve, to some degree, most elements and compounds. The major components of TDS of natural waters include: bicarbonate (HCO3-), calcium (Ca+2), sulfate (SO4-2), hydrogen (H+), silica (SiO4), chlorine (Cl-), magnesium (Mg+2), sodium (Na+), potassium (K+), nitrogen (N2, NH3, NO-2, NO-3), and phosphorus in the form of phosphate (PO4-3). The have been listed more or less in order from most concentrated to least concentrated in typical waterways. Bicarbonate can make up 50% of TDS in some streams. Minor constituents that are normally just a trace in streams include: iron (Fe+3), copper (Cu+2), zinc (Zn+), boron (B+3), manganese (Mn+2), and molybdenum (Mo+).

A constant level of these total dissolved solids is essential for the maintenance of aquatic life because the density of total solids determines flow of water in and out of an organism's cells (osmosis). Plus the nutrients (nitrogen and phosphorus) are important for organism growth. A sudden or extreme change in TDS can be detrimental to aquatic life. For instance, an increase in salts could kill freshwater species whose bodies are not constructed to live in saltwater.

The natural sources of dissolved solids are rocks, bedrock, and soils. As water comes in contact with them, minerals will dissolve to some degree. Geologic settings that include limestone (calcium carbonate) and halite (salt, sodium chloride), which readily dissolve in water, generally have waters with high TDS values. Regions underlain by rocks not susceptible to weathering, such as quartz-rich granite, generally have waters with low TDS levels.

The hydrological setting also exerts a strong control on the amount of TDS. Groundwater generally has high TDS values because it moves slowly and comes in contact with lots of rocks and sediment. Conversely, storm water runoff has low TDS because it moves rapidly and has limited contact with rocks and sediments. Because of this relationship, TDS is typically highest in streams flowing during low flow conditions, when groundwater is the primary source of water. During high flow conditions, stream TDS is low because storm runoff is the primary source of water.

The greater the land area that water has to come in contact with soils and rocks, the more likely the TDS levels will be higher. For instance, if TDS levels were analyzed at the mouth of a stream that drained a 60 square mile watershed, they would higher than a sample taken from the mouth of a stream draining the upper 5 square miles of that same watershed.

The term total dissolved solids is often confused with other types of stream measurement tests. For instance, total solids (TS) is the sum of both TDS and visible solids (sediments) that would contribute to the turbidity of water. Total dissolved solids is also different than conductivity, which is a measure of the electrical conductance of water. Total dissolved solids measures the amount of ions in water, while conductivity measures those ions' ability to do something - conduct electricity. Distilled water (very low TDS) has little capacity for electron conductivity. The more ions in the water, the higher the electron flow. Usually there is a strong correlation between conductivity and TDS, but there is still a difference between the two. Conductivity is only an approximate predictor of TDS. Salinity is also different than total dissolved solids. Salinity deals only with salts and is defined as the concentration of all ionic constituents that include halides, bicarbonates, and sodium chloride.

Total dissolved solids is a complex water quality parameter because it is directly linked to so many chemical and biological processes, and incorporates a number of the other water parameters that we test. The average concentration of total dissolved solids for the world's rivers is 100mg/L, while North American rivers average 142.6 mg/L. Each region in the country has a specific, normal TDS level - some high, some low. Ecosystems are adjusted to local conditions; therefore, a large change in TDS concentrations will disrupt the system and increase its overall sensitivity.

Human Impacts

Because total dissolved solids are related to so many other chemical parameters and processes, humans can increase TDS levels in a number of ways. Rain will wash additional dissolved solids into a stream. This will occur naturally but we can increase it by encouraging soil erosion through poor farming practices, construction sites, timbering, and removal of riparian zones. Soil can bring in more minerals, nutrients, and metals. Rain will also be more effective at dissolving minerals if it is acidic, and humans have drastically lowered the pH (increased acidity) of rainfall through car exhausts and the burning of fossil fuels. Rain can also wash excess nutrients into a stream if too much fertilizer is placed on cropland or lawns. Since nutrients are a component of TDS, any human practices that contribute nutrients to streams will increase TDS levels. See the Nitrogen and Phosphorus Information Sheets. An easy to understand dissolved solid - salt - is another common artificial source of higher TDS. Road salt used on roads in the winter can wash into waterways.

Water Quality Criteria

The Environmental Protection Agency (EPA) limits the amount of dissolved solids to 500 mg/L for drinking water. Each component that makes up TDS has a specific water quality limit also. A monthly average of 500 mg/L and a maximum of 750 mg/L for a single event is considered safe for aquatic life. Example Total Dissolved Solids Data - French Creek The Pennsylvania Department of Environmental Protection (DEP) has systematically collected water quality data from French Creek, at Meadville since about 1973. A summary of these data is below to provide an understanding of past values of total dissolved solids and to demonstrate relationships of past TDS levels with discharge.

Average: 202.00 mg/L

Maximum: 454.00 mg/L

Minimum: 68.00 mg/L