TDS and Electrical Conductivity - Lenntech
Salinity and total dissolved solids calculations are derived from conductivity. mass fraction, and it ensures that all thermodynamic relationships (density, sound . ionised solutes or total dissolved solid (TDS) content. The relationship between conductivity and TDS is not directly linear, however, since the conductive. Abstract. Conductivity (EC) and total dissolved solids (TDS) are water quality parameters, which are used to describe salinity level. These two parameters are .
TDS is used to calculate the volume of sludge that will be generated by treating the mine-water e. This parameter is thus essential for the dimensioning of storage basins for sludge deposits. Rarely is TDS being determined directly but rather is being estimated by different means, for which the conductivity method is but one option DWA, In their dataset, they included the 'total dissolved solids imbalance', which is based on the measured and calculated TDS from the chemical analysis.
Van Niekerk et al. None of these studies explicitly investigated mine-waters. Therefore, this study aims at filling this gap. Electrical conductivity Basics Although practitioners and researchers perform electrical conductivity measurements on a regular basis, they do not always know the theoretical background. Therefore, the following sections will provide a short introduction to the subject.
Only the most relevant equations will be given in helping to understand this relationship. The electrical conductivity of an aqueous solution is its ability to pass an electric current. Being a characteristic property of solutions, it is used in a range of fields and industrial applications, e. Electrical conductivity depends mainly on the concentration of dissolved electrolytes and gases.
With respect to the dissociated ions in particular, EC is a function of their mobility, determined by charge and dissolved radius and the solution's viscosity and temperature Coury, The parameter measured by the instrument is the conductance G, being the reciprocal of the resistance R. Resistance R is defined by the current I that flows when a certain potential U is applied. G, the electrical conductance, depends on the properties of the solution as well as on the dimensions of the measuring cell.
Those dimensions, the physical shape and the size of the conductivity cell limiting the ions' movement, are represented by the cell constant K, which is constituted of the electrode distance l and the electrode surface A. The latter should be avoided, as it is not an SI unit. Different authors assume that the terms specific conductance Atekwana et al.
Assumption that a temperature compensation was done can thus lead to misinterpretation of data. Before measurements are performed, instrument calibration with a standard solution of known electrical conductivity ks is necessary Gelhaus and Lacourse, This calibration determines the cell constant for the measured electrical conductance of that standard GS.
Conductivity, Salinity & Total Dissolved Solids
Subsequently, the cell constant will be used to convert conductance to conductivity, because although in theory the cell constant is defined by its geometry, it is practically only measurable under working conditions and also changes over time. Due to the large dependency of EC on temperature, values are only comparable if either measured at or converted to the same reference temperature Smith, It characterizes the rate of conductivity change per degree of temperature change.
For practicality, mean temperature coefficients for certain temperature ranges are used Smith, In case of a linear correction, the same coefficient is used across the whole temperature range. Many conductivity meters have a built-in temperature compensation, using mainly linear algorithms to convert the sample conductivity to the chosen reference temperature.
Calculation of EC from ionic concentrations If results from a chemical analysis are available, electrical conductivity can be computed from the ions' concentrations. Twelve different equations have been proposed and are discussed in McCleskey et al. The method used in this paper is based on Rossum and Couryapplying an extended table listed in Wolkersdorfer At the assumed state of infinite dilution, the contribution of different types of ions in a complex solution to conductivity is additive.
For accurate EC calculations, it is therefore imperative to account for the individual concentrations of the ions in solution. Those results can then be used to verify the accuracy of the chemical analysis, a method that is more reliable than the commonly-used ionic balance Wolkersdorfer, It includes all inorganic and organic dissociated anions and cations as well as undissociated dissolved species McNeil and Cox, It is sometimes wrongly called total dissolved salts Hobbs and Cobbing, ; Hobbs et al.
Sometimes it is also defined as the content of dissolved inorganic salts DWAF, b. The standard method for the determination of TDS is gravimetric, where an accurately measured volume of filtered sample is evaporated and dried at a certain temperature e. South African National Standard, The volume of sample to be evaporated depends on the expected TDS, which can be estimated from a quick EC measurement.
If the sample volume chosen is too large, an excessive amount of residue can remain and water entrapped in the crust might not be released in its entirety during the drying process Rainwater and Thatcher, Though the determination of TDS seems to be relatively straightforward, the results of TDS determination are influenced by various parameters, such as pore size, porosity and filter thickness, particle size, amount of sample and drying time as well as method used Table 1.
Some organic and also inorganic matter, such as nitrite and boron, will be volatilized Howard, ; Hem,but not all of it will be removed completely. Especially at low pH, some anion content, such as chloride and nitrite, may be lost by the volatilization of acids Standard Methods for the Examination of Water and Wastewater, Contrary to those losses, some weight gain can also happen due to oxidation or transformation into hydroxides.
Some parameters may cause a longer drying time. These are a particularly high mineral content, a high bicarbonate concentration or large concentrations of calcium, magnesium, chloride and sulphate.
Weighing will have to be done as soon as possible after the required cooling phase, in a desiccator US Environmental Protection Agency, Even after the slower release of contained water, some crystallization water may remain in residue when rich in gypsum Hem, At lower temperatures, not all of those processes occur until completion, usually leading to more water retained in the solids.
Yet, different temperatures are recommended for drying by different standards Table 1. In addition to the results, it is consequently necessary to record the drying temperature - even for more dilute natural waters for which differences in the drying temperature yield not substantially different results Hem, Because the evaporation method is time consuming and thus expensive, methods for determining TDS mathematically from ion concentrations, sulphate concentration and EC exist.
If a reasonably complete chemical analysis was performed, a possible method for determining TDS is to add up all of the ion concentrations, which usually yields results quite close to the gravimetric method. To avoid errors in the TDS calculation, those concentrations have to be added to the main ions. Because almost all the conductivity is accounted for by the dissolved ions, there is a direct proportionality between EC and TDS: At lower concentrations, the relation between concentration and EC for single electrolyte solutions is linear and flattens off for higher ones because the ionic mobility decreases with increasing concentration due to interferences and interactions between the ions.
Yet, the slope of the linear part, as well as the degree of flattening off at higher concentrations, differs for different dissolved electrolytes. As natural waters are not simple solutions and contain various ionic and undissociated species with widely varying amounts and proportions, the relationship between EC and TDS becomes complicated. Water hardness is the measurement of the amount of ions which have lost two electrons divalent cations dissolved in the tested water and is therefore, related to total dissolved solids.
The more divalent cations dissolved in the water the "harder" the water. Generally the most common divalent cations are calcium and magnesium, however other divalent cations may contribute including iron, strontium, aluminum, and manganese. Typically the other divalent cations contribute little to no appreciable additions to the water hardness measurement. A stream or river's hardness reflects the geology of the catchment's area and sometimes provides a measure of the influence of human activity in a watershed.
For example, sites that have active or abandoned mines nearby often have higher concentrations of iron ions in the water resulting in a very high hardness degree. General conversions are below: Why is Conductivity Important? Factors that affect water volume like heavy rain or evaporation affect conductivity. Runoff or flooding over soils that are high in salts or minerals can cause a spike in conductivity despite the increase in water flow.
Conductivity, in particular specific conductance, is one of the most useful and commonly measured water quality parameters 3. In addition to being the basis of most salinity and total dissolved solids calculations, conductivity is an early indicator of change in a water system. Most bodies of water maintain a fairly constant conductivity that can be used as a baseline of comparison to future measurements 1.
Significant change, whether it is due to natural flooding, evaporation or man-made pollution can be very detrimental to water quality. Seawater cannot hold as much dissolved oxygen as freshwater due to its high salinity.
Conductivity and salinity have a strong correlation 3. As conductivity is easier to measure, it is used in algorithms estimating salinity and TDS, both of which affect water quality and aquatic life.
Salinity is important in particular as it affects dissolved oxygen solubility 3. The higher the salinity level, the lower the dissolved oxygen concentration.
This means that, on average, seawater has a lower dissolved oxygen concentration than freshwater sources. Aquatic Organism Tolerance Euryhaline including anadromous and catadromous species have the widest salinity tolerance range as they travel between both saltwater and freshwater. Most aquatic organisms can only tolerate a specific salinity range The physiological adaption of each species is determined by the salinity of its surrounding environment. Most species of fish are stenohaline, or exclusively freshwater or exclusively saltwater However, there are a few organisms that can adapt to a range of salinities.
These euryhaline organisms can be anadromous, catadromous or true euryhaline. Anadromous organisms live in saltwater but spawn in freshwater. Catadromous species are the opposite — they live in freshwater and migrate to saltwater to spawn True euryhaline species can be found in saltwater or freshwater at any point in their life cycle Estuarine organisms are true euryhaline.
Euryhaline species live in or travel through estuaries, where saline zonation is evident.
Conductivity, Salinity & Total Dissolved Solids - Environmental Measurement Systems
Salinity levels in an estuary can vary from freshwater to seawater over a short distance While euryhaline species can comfortably travel across these zones, stenohaline organisms cannot and will only be found at one end of the estuary or the other.
Species such as sea stars and sea cucumbers cannot tolerate low salinity levels, and while coastal, will not be found within many estuaries Some aquatic organisms can even be sensitive to the ionic composition of the water. An influx of a specific salt can negatively affect a species, regardless of whether the salinity levels remain within an acceptable range Most aquatic organisms prefer either freshwater or saltwater.
Few species traverse between salinity gradients, and fewer still tolerate daily salinity fluctuations. Salinity tolerances depend on the osmotic processes within an organism.
Fish and other aquatic life that live in fresh water low-conductivity are hyperosmotic Thus these organisms maintain higher internal ionic concentrations than the surrounding water On the other side of the spectrum, saltwater high-conductivity organisms are hypoosmotic and maintain a lower internal ionic concentration than seawater.
Euryhaline organisms are able to adapt their bodies to the changing salt levels. Each group of organisms has adapted to the ionic concentrations of their respective environments, and will absorb or excrete salts as needed Altering the conductivity of the environment by increasing or decreasing salt levels will negatively affect the metabolic abilities of the organisms.
Even altering the type of ion such as potassium for sodium can be detrimental to aquatic life if their biological processes cannot deal with the different ion Conductivity Change can Indicate Pollution Oil or hydrocarbons can reduce the conductivity of water.
Lamiot via Wikimedia Commons A sudden increase or decrease in conductivity in a body of water can indicate pollution. Agricultural runoff or a sewage leak will increase conductivity due to the additional chloride, phosphate and nitrate ions 1. An oil spill or addition of other organic compounds would decrease conductivity as these elements do not break down into ions In both cases, the additional dissolved solids will have a negative impact on water quality.
Salinity affects water density. The higher the dissolved salt concentration, the higher the density of water 4. The increase in density with salt levels is one of the driving forces behind ocean circulation When sea ice forms near the polar regions, it does not include the salt ions.
Instead, the water molecules freeze, forcing the salt into pockets of briny water This brine eventually drains out of the ice, leaving behind an air pocket and increasing the salinity of the water surrounding the ice. As this saline water is denser than the surrounding water, it sinks, creating a convection pattern that can influence ocean circulation for hundreds of kilometers Conductivity and salinity vary greatly between different bodies of water. Most freshwater streams and lakes have low salinity and conductivity values.
The oceans have a high conductivity and salinity due to the high number of the dissolved salts present. Freshwater Conductivity Sources Many different sources can contribute to the total dissolved solids level in water. In streams and rivers, normal conductivity levels come from the surrounding geology 1. Clay soils will contribute to conductivity, while granite bedrock will not 1.
The minerals in clay will ionize as they dissolve, while granite remains inert. Likewise, groundwater inflows will contribute to the conductivity of the stream or river depending on the geology that the groundwater flows through. Groundwater that is heavily ionized from dissolved minerals will increase the conductivity of the water into which it flows. Saltwater Conductivity Sources Most of the salt in the ocean comes from runoff, sediment and tectonic activity Rain contains carbonic acid, which can contribute to rock erosion.
As rain flows over rocks and soil, the minerals and salts are broken down into ions and are carried along, eventually reaching the ocean Hydrothermal vents along the bottom of the ocean also contribute dissolved minerals As hot water seeps out of the vents, it releases minerals with it.
Submarine volcanoes can spew dissolved minerals and carbon dioxide into the ocean The dissolved carbon dioxide can become carbonic acid which can erode rocks on the surrounding seafloor and add to the salinity.
As water evaporates off the surface of the ocean, the salts from these sources are left behind to accumulate over millions of years Discharges such as pollution can also contribute to salinity and TDS, as wastewater effluent increases salt ions and an oil spill increases total dissolved solids 1.
When does Conductivity Fluctuate? Water flow and water level changes can also contribute to conductivity through their impact on salinity. Water temperature can cause conductivity levels to fluctuate daily. In addition to its direct effect on conductivity, temperature also influences water density, which leads to stratification.
Stratified water can have different conductivity values at different depths. Water flow, whether it is from a spring, groundwater, rain, confluence or other sources can affect the salinity and conductivity of water. Likewise, reductions in flow from dams or river diversions can also alter conductivity levels Water level changes, such as tidal stages and evaporation will cause salinity and conductivity levels to fluctuate as well.
Conductivity and Temperature Conductivity is temperature dependent. When water temperature increases, so will conductivity 3. Temperature affects conductivity by increasing ionic mobility as well as the solubility of many salts and minerals This can be seen in diurnal variations as a body of water warms up due to sunlight, and conductivity increases and then cools down at night decreasing conductivity.
This standardized reporting method is called specific conductance 1. Seasonal variations in conductivity, while affected by average temperatures, are also affected by waterflow.
In some rivers, as spring often has the highest flow volume, conductivity can be lower at that time than in the winter despite the differences in temperature In water with little to no inflow, seasonal averages are more dependent on temperature and evaporation.
Conductivity and Water Flow The effect of water flow on conductivity and salinity values is fairly basic. If the inflow is a freshwater source, it will decrease salinity and conductivity values Freshwater sources include springs, snowmelt, clear, clean streams and fresh groundwater On the other side of the spectrum, highly mineralized groundwater inflows will increase conductivity and salinity 1.