What is Conductivity? 什么是电导率？

Conductivity is a measure of water’s capability to pass electrical flow. This ability is directly related to the concentration of ions in the water ¹. These conductive ions come from dissolved salts and inorganic materials such as alkalis, chlorides, sulfides and carbonate compounds ³. Compounds that dissolve into ions are also known as electrolytes ⁴⁰. The more ions that are present, the higher the conductivity of water. Likewise, the fewer ions that are in the water, the less conductive it is. Distilled or deionized water can act as an insulator due to its very low (if not negligible) conductivity value ². Sea water, on the other hand, has a very high conductivity.
电导率是水通过电流能力的量度。这种能力与水中离子的浓度直接相关¹ 。这些导电离子来自溶解的盐和无机材料，例如碱金属、氯化物、硫化物和碳酸盐化合物³ 。溶解成离子的化合物也称为电解质⁴⁰ 。存在的离子越多，水的电导率越高。同样，水中的离子越少，其导电性就越差。蒸馏水或去离子水由于其非常低（如果不可忽略）的电导率值²可以充当绝缘体。另一方面，海水具有非常高的电导率。

Ions conduct electricity due to their positive and negative charges ¹. When electrolytes dissolve in water, they split into positively charged (cation) and negatively charged (anion) particles. As the dissolved substances split in water, the concentrations of each positive and negative charge remain equal. This means that even though the conductivity of water increases with added ions, it remains electrically neutral ².
离子因其正电荷和负电荷而导电¹ 。当电解质溶解在水中时，它们分裂成带正电（阳离子）和带负电（阴离子）的颗粒。当溶解的物质在水中分裂时，每个正电荷和负电荷的浓度保持相等。这意味着即使水的电导率随着离子的添加​​而增加，它仍保持电中性² 。

 

Conductivity Units 电导率单位

Conductivity is usually measured in micro- or millisiemens per centimeter (uS/cm or mS/cm). It can also be reported in micromhos or millimhos/centimeter (umhos/cm or mmhos/cm), though these units are less common. One siemen is equal to one mho ¹. Microsiemens per centimeter is the standard unit for freshwater measurements. Reports on seawater conductivity use micro-, milli- and and sometimes even just siemen/mho per centimeter, depending on the publication.
电导率通常以微西门子或毫西门子每厘米（uS/cm 或 mS/cm）为单位进行测量。也可以用微欧姆或毫欧/厘米（umhos/cm 或 mmhos/cm）来报告，尽管这些单位不太常见。一西门子等于一姆欧¹ 。微西门子每厘米是淡水测量的标准单位。关于海水电导率的报告使用微、毫，有时甚至仅使用西门子/姆欧每厘米，具体取决于出版物。

 

Specific Conductance 比电导

Specific conductance is a conductivity measurement made at or corrected to 25° C ³. This is the standardized method of reporting conductivity. As the temperature of water will affect conductivity readings, reporting conductivity at 25° C allows data to be easily compared ³. Specific conductance is usually reported in uS/cm at 25° C ⁶.
比电导是在25℃下进行或校正至25℃ ^{3 的}电导率测量。这是报告电导率的标准化方法。由于水温会影响电导率读数，因此报告 25° C 时的电导率可以轻松比较数据³ 。电导率通常以 uS/cm 为单位（25°C ^{6 ）} 。

If a conductivity measurement is made at 25° C, it can simply be reported as the specific conductance. If a measurement is made at a different temperature and corrected to 25° C, then the temperature coefficient must be considered. The specific conductance temperature coefficient can range depending on the measured temperature and ionic composition of the water ³². A coefficient of 0.0191-0.02 is commonly used based on KCl standards ^3,32. NaCl-based solutions should have a temperature coefficient of 0.02-0.0214 ³³.
如果在 25°C 下进行电导率测量，则可以简单地将其报告为比电导。如果在不同温度下进行测量并校正至 25° C，则必须考虑温度系数。比电导温度系数的范围可取决于测量的温度和水³²的离子组成。根据 KCl 标准，通常使用 0.0191-0.02 的系数^3,32 。氯化钠溶液的温度系数应为0.02-0.0214 ³³ 。

 

Resistivity 电阻率

Conductivity is formally defined as the reciprocal of resistivity, which is worth elaborating on ³. Resistivity is a measurement of water’s opposition to the flow of a current over distance. Pure water has a resistance of 18.2 Mohm*cm ⁵. Resistivity decreases as the ionic concentration in water increases. A fun way to remember that resistivity and conductivity are reciprocals (1/measurement) is in the unit name – mho and ohm are the same letters, in reverse.
电导率的正式定义是电阻率的倒数，值得详细阐述³ 。电阻率是水对远距离电流流动的阻力的测量。纯水的电阻为18.2Mohm*cm ⁵ 。电阻率随着水中离子浓度的增加而降低。记住电阻率和电导率是倒数（1/测量值）的一个有趣方法是在单位名称中 - mho 和 ohm 是相同的字母，方向相反。

 

Conductance 电导

Conductance is part of conductivity, but it is not a specific measurement on its own. Electrical conductance is dependent on the length of the conductor, just as resistance is ¹⁸. Conductance is measured in mhos or siemens ¹⁹. Conductivity is the conductance (S) measured across a specified distance (1 cm), which is incorporated into the units (S/cm) ¹⁹. As such, the conductance of water will change with the distance specified. But as long as the temperature and composition remains the same, the conductivity of water will not change.
电导是电导率的一部分，但它本身并不是一种特定的测量。电导率取决于导体的长度，就像电阻为¹⁸一样。电导以姆欧或西门子为单位测量¹⁹ 。电导率是在指定距离 (1 cm) 上测得的电导 (S)，并纳入单位 (S/cm) ¹⁹中。因此，水的电导率将随着指定的距离而变化。但只要温度和成分保持不变，水的电导率就不会改变。

 

What is Salinity? 什么是盐度？

Salinity is an ambiguous term. As a basic definition, salinity is the total concentration of all dissolved salts in water ⁴. These electrolytes form ionic particles as they dissolve, each with a positive and negative charge. As such, salinity is a strong contributor to conductivity. While salinity can be measured by a complete chemical analysis, this method is difficult and time consuming ¹³. Seawater cannot simply be evaporated to a dry salt mass measurement as chlorides are lost during the process ²⁶.
盐度是一个含糊不清的术语。作为基本定义，盐度是水中所有溶解盐的总浓度⁴ 。这些电解质在溶解时形成离子颗粒，每个颗粒都带有正电荷和负电荷。因此，盐度对电导率有很大影响。虽然盐度可以通过完整的化学分析来测量，但这种方法既困难又耗时¹³ 。海水不能简单地蒸发至干盐质量测量，因为氯化物在该过程中会损失²⁶ 。

More often, salinity is not measured directly, but is instead derived from the conductivity measurement ⁶. This is known as practical salinity. These derivations compare the specific conductance of the sample to a salinity standard such as seawater ⁶. Salinity measurements based on conductivity values are unitless, but are often followed by the notation of practical salinity units (psu) ²⁵.
更常见的是，盐度不是直接测量的，而是通过电导率测量得出的⁶ 。这称为实际盐度。这些推导将样品的电导率与盐度标准（例如海水）进行比较⁶ 。基于电导率值的盐度测量是无单位的，但通常后面带有实际盐度单位 (psu) ²⁵的符号。

There are many different dissolved salts that contribute to the salinity of water. The major ions in seawater (with a practical salinity of 35) are: chloride, sodium, magnesium, sulfate, calcium, potassium, bicarbonate and bromine ²⁵. Many of these ions are also present in freshwater sources, but in much smaller amounts ⁴. The ionic compositions of inland water sources are dependent on the surrounding environment. Most lakes and rivers have alkali and alkaline earth metal salts, with calcium, magnesium, sodium, carbonates and chlorides making up a high percentage of the ionic composition ⁴. Freshwater usually has a higher bicarbonate ratio while seawater has greater sodium and chloride concentrations ³⁹.
有许多不同的溶解盐会导致水的盐度增加。海水（实际盐度为 35）中的主要离子是：氯离子、钠离子、镁离子、硫酸根离子、钙离子、钾离子、碳酸氢根离子和溴离子²⁵ 。其中许多离子也存在于淡水源中，但含量要少得多⁴ 。内陆水源的离子组成取决于周围环境。大多数湖泊和河流都含有碱金属盐和碱土金属盐，其中钙、镁、钠、碳酸盐和氯化物在离子成分中所占比例很高⁴ 。淡水通常具有较高的碳酸氢盐比率，而海水具有较高的钠和氯化物浓度³⁹ 。

 

Absolute Salinity 绝对盐度

While the Practical Salinity Scale is acceptable in most situations, a new method of salinity measurement was adopted in 2010. This method, called TEOS-10, determines absolute salinity as opposed to the practical salinity derived from conductivity. Absolute salinity provides an accurate and consistent representation of the thermodynamic state of the system ²⁴. Absolute salinity is both more accurate and more precise than practical salinity and can be used to estimate salinity not only across the ocean, but at greater depths and temperature ranges ²⁴. TEOS-10 is derived from a Gibbs function, which requires more complex calculations, but offers more useful information ²⁴.
虽然实用盐度等级在大多数情况下是可以接受的，但 2010 年采用了一种新的盐度测量方法。这种方法称为 TEOS-10，可确定绝对盐度，而不是根据电导率得出的实际盐度。绝对盐度提供系统²⁴的热力学状态的准确且一致的表示。绝对盐度比实际盐度更准确、更精确，不仅可用于估计整个海洋的盐度，而且可用于估计更大深度和温度范围的盐度²⁴ 。 TEOS-10 源自吉布斯函数，它需要更复杂的计算，但提供了更有用的信息²⁴ 。

 

Salinity Units 盐度单位

The units used to measure salinity fluctuate based on application and reporting procedure. Parts per thousand or grams/kilogram (1 ppt = 1 g/kg) used to be the standard ²². In some freshwater sources, this is reported in mg/L ^{4, 37}. Now salinity values are reported based on the unitless Practical Salinity Scale (sometimes denoted in practical salinity units as psu) ²². As of 2010, an Absolute Salinity calculation was developed, but is not used for database archives ²⁴. Absolute salinity is reported in g/kg and is denoted by the symbol S_A. TEOS-10 offers pre-programmed equations to calculate absolute salinity.
用于测量盐度的单位根据应用和报告程序而波动。千分之几或克/千克（1 ppt = 1 g/kg）曾经是标准²² 。在一些淡水源中，以 mg/L ^{4, 37}为单位报告。现在，盐度值是根据无单位实用盐度等级（有时以实用盐度单位表示为 psu） ²²来报告的。截至 2010 年，开发了绝对盐度计算，但未用于数据库档案²⁴ 。绝对盐度以 g/kg 为单位报告，并用符号_SA表示。 TEOS-10 提供预编程方程来计算绝对盐度。

The units psu, ppt and S_A g/kg are nearly equivalent (and often interchanged) ⁶. All three methods are based on an approximate salinity value of 35 in seawater ²⁴. However, there are some distinctions that must be made.
单位 psu、ppt 和 S _A g/kg 几乎相等（并且经常互换） ⁶ 。所有三种方法均基于海水²⁴中的近似盐度值 35。然而，必须做出一些区分。

Practical salinity units are dimensionless and are based on conductivity studies of potassium chloride solutions and seawater ¹³. These studies were done with 32.4356 g/kg KCL solution and “Copenhagen water” which has a chlorinity of 19.374 ppt ²⁵. This north Atlantic sea water was given a set practical salinity of 35 psu ²⁵. The practical salinity scale is considered accurate for values between 2 and 42 psu ²⁶. These are the most common units used, and practical salinity remains the most common salinity value stored for data archives ²⁴.
实用的盐度单位是无量纲的，并且基于氯化钾溶液和海水的电导率研究¹³ 。这些研究是使用 32.4356 g/kg KCL 溶液和氯度为 19.374 ppt ²⁵的“哥本哈根水”进行的。该北大西洋海水的实际盐度设定为 35 psu ²⁵ 。对于 2 到 42 psu ²⁶之间的值，实际盐度等级被认为是准确的。这些是最常用的单位，并且实际盐度仍然是为数据档案存储的最常见的盐度值²⁴ 。

The historical definition of salinity was based on chloride concentration (which could be determined by titration) ²⁸. This calculation used the following equation:
历史上盐度的定义是基于氯化物浓度（可以通过滴定确定） ²⁸ 。该计算使用以下等式：

This method is only acceptable for seawater, as it is limited in estuaries, brackish and freshwater sources ²⁸. While salinity and chlorinity are proportional in seawater, equations based on this are not accurate in freshwater or when chlorinity ratios change ²⁶.
该方法仅适用于海水，因为它在河口、咸水和淡水源中受到限制²⁸ 。虽然海水中的盐度和氯度成正比，但基于此的方程在淡水中或当氯度比发生变化时并不准确²⁶ 。

Absolute salinity in g/kg is best for studies that require very precise data. It is consistent with other SI units as a true mass fraction, and it ensures that all thermodynamic relationships (density, sound, speed and heat capacity) remain consistent ²⁴. These units also help determine specific ions’ contributions to salinity values ³⁹. Absolute salinity also offers a greater range and more accurate values than other salinity methods when ionic composition is known ²⁴.
以 g/kg 为单位的绝对盐度最适合需要非常精确数据的研究。它与其他国际单位制的真实质量分数一致，并且确保所有热力学关系（密度、声音、速度和热容）保持一致²⁴ 。这些单位还有助于确定特定离子对盐度值的贡献³⁹ 。当离子组成已知时，绝对盐度还提供比其他盐度方法更大的范围和更准确的值²⁴ 。

 

What are Total Dissolved Solids?
什么是总溶解固体？

Total dissolved solids (TDS) combine the sum of all ion particles that are smaller than 2 microns (0.0002 cm) ¹¹. This includes all of the disassociated electrolytes that make up salinity concentrations, as well as other compounds such as dissolved organic matter. In “clean” water, TDS is approximately equal to salinity ¹². In wastewater or polluted areas, TDS can include organic solutes (such as hydrocarbons and urea) in addition to the salt ions ¹².
总溶解固体 (TDS) 是所有小于 2 微米 (0.0002 cm) ¹¹的离子颗粒的总和。这包括构成盐浓度的所有解离电解质，以及其他化合物，例如溶解的有机物。在“清洁”水中，TDS 大约等于盐度¹² 。在废水或污染区域，TDS 除了盐离子之外还可能包含有机溶质（例如碳氢化合物和尿素） ¹² 。

While TDS measurements are derived from conductivity, some states, regions and agencies often set a TDS maximum instead of a conductivity limit for water quality ³⁷. At most, freshwater can have 2000 mg/L of total dissolved solids, and most sources should have much less than that ¹³. Depending on the ionic properties, excessive total dissolved solids can produce toxic effects on fish and fish eggs. Salmonids exposed to higher than average levels of CaSO4 at various life stages experienced reduced survival and reproduction rates ³⁷. When total dissolved solids ranged above 2200-3600 mg/L, salmonids, perch and pike all showed reduced hatching and egg survival rates ³⁷.
虽然 TDS 测量值是根据电导率得出的，但一些州、地区和机构经常设置 TDS 最大值，而不是水质的电导率限值³⁷ 。淡水最多可含有 2000 mg/L 的总溶解固体，而大多数水源的含量应远低于此¹³ 。根据离子特性，过量的总溶解固体会对鱼和鱼卵产生毒性作用。在不同生命阶段暴露于高于平均水平的 CaSO4 的鲑鱼的存活率和繁殖率都会降低³⁷ 。当总溶解固体含量高于 2200-3600 mg/L 时，鲑鱼、鲈鱼和梭子鱼的孵化率和卵存活率均下降³⁷ 。

Dissolved solids are also important to aquatic life by keeping cell density balanced ¹¹. In distilled or deionized water, water will flow into an organism’s cells, causing them to swell ¹¹. In water with a very high TDS concentration, cells will shrink. These changes can affect an organism’s ability to move in a water column, causing it to float or sink beyond its normal range ¹¹.

TDS can also affect water taste, and often indicates a high alkalinity or hardness ¹².

 

TDS Units

Total dissolved solids are reported in mg/L. TDS can be measured by gravimetry (with an evaporation dish) or calculated by multiplying a conductivity value by an empirical factor ¹³. While TDS determination by evaporation is more time-consuming, it is useful when the composition of a water source is not known. Deriving TDS from conductivity is quicker and suited for both field measurements and continuous monitoring ⁴².

When calculating total dissolved solids from a conductivity measurement, a TDS factor is used. This TDS constant is dependent on the type of solids dissolved in water, and can be changed depending on the water source. Most conductivity meters and other measurement options will use a common, approximated constant around 0.65 ³². However, when measuring mixed water or saline water (with a conductivity value greater than 5000 uS/cm), the TDS constant should be higher: around 0.735 and 0.8 respectively ²⁰. Likewise, fresh or nearly pure water should have a lower TDS constant closer to 0.47-0.50 ³⁶.

Standard Methods for the Examination of Water and Wastewater accepts a TDS constant of 0.55-0.7, though if the water source is known to be high in calcium or sulfate ions, a constant of 0.8 may be used ¹³.  Several conductivity meters will accept a constant outside of this range, but it is recommended to reanalyze the sample by evaporation to confirm this ratio ¹³.

As seen in the table below, solutions with the same conductivity value, but different ionic constitutions (KCl vs NaCl vs 442) will have different total dissolved solid concentrations. This is due to the difference in molecular weight ⁴⁰.  In addition, the ionic composition will change the recommended TDS constant.

All three standards are acceptable for conductivity calibrations. However, the ionic composition should be considered if calculating total dissolved solids. If a project allows for it, the TDS constant should be determined for each specific site based on known ionic constituents in the water ⁶.

 

Why is Conductivity Important?

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.

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. Oxygen is about 20% less soluble in seawater than in freshwater at the same temperature ³. This means that, on average, seawater has a lower dissolved oxygen concentration than freshwater sources. The effect of salinity on the solubility of dissolved gases  is due to Henry’s Law; the constant used will changes based on salt ion concentrations ³⁹.

 

Aquatic Organism Tolerance

Most aquatic organisms can only tolerate a specific salinity range ¹⁴. 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 ⁴³. 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 ⁴³. True euryhaline species can be found in saltwater or freshwater at any point in their life cycle ⁴³. Estuarine organisms are true euryhaline.

Euryhaline species live in or travel through estuaries, where saline zonation is evident. Salinity levels in an estuary can vary from freshwater to seawater over a short distance ²¹. 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 ²¹. 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 ¹⁴.

Salinity tolerances depend on the osmotic processes within an organism. Fish and other aquatic life that live in fresh water (low-conductivity) are hyperosmotic ¹⁵. Hyperosmotic defines a cell’s ability to eliminate water and retain ions. Thus these organisms maintain higher internal ionic concentrations than the surrounding water ¹⁶. 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 ¹⁶. 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 ¹⁴.

 

Conductivity Change Can Indicate Pollution

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 ³⁴. In both cases, the additional dissolved solids will have a negative impact on water quality.

 

Salinity Contributes to Ocean Convection

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 ²². 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 ²². 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 ²².

 

Where do TDS and Salinity Come From?

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

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 ¹⁷. 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 ¹⁷. Hydrothermal vents along the bottom of the ocean also contribute dissolved minerals ¹⁷. 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 ¹⁷. 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 ²⁷.

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?

Conductivity is dependent on water temperature and salinity/TDS ³⁸. 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 ²⁹. Water level changes, such as tidal stages and evaporation will cause salinity and conductivity levels to fluctuate as well.

 

Conductivity and Temperature

When water temperature increases, so will conductivity ³. For every 1°C increase, conductivity values can increase 2-4% ³. Temperature affects conductivity by increasing ionic mobility as well as the solubility of many salts and minerals ³⁰. 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).

Due to temperature’s direct effect, conductivity is measured at or corrected to a standardized temperature (usually 25°C) for comparability. 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 ²³. 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 ²⁹. Freshwater sources include springs, snowmelt, clear, clean streams and fresh groundwater ²¹. On the other side of the spectrum, highly mineralized groundwater inflows will increase conductivity and salinity ¹. Agricultural runoff, in addition to being high in nutrients, often has a higher concentration of dissolved solids that can influence conductivity ²³. For both freshwater and mineralized water, the higher the flow volume, the more it will affect salinity and conductivity ²⁹.

Rain itself can have a higher conductivity than pure water due to the incorporation of gases and dust particles ²³. However, heavy rainfall can decrease the conductivity of a body of water as it dilutes the current salinity concentration ²⁹.

If heavy rainfall or another major weather event contributes to flooding, the effect on conductivity depends on the water body and surrounding soil. In areas with dry and wet seasons, conductivity usually drops overall during the wet season due to the dilution of the water source ⁴⁴. Though the overall conductivity is lower for the season, there are often conductivity spikes as water initially enters a floodplain. If a floodplain contains nutrient-rich or mineralized soil, previously dry salt ions can enter solution as it is flooded, raising the conductivity of water ⁴⁴.

If coastal water floods, the opposite effect can occur. Though turbidity will increase, the conductivity of water often decreases during a coastal flood ⁴⁵. Seawater will pick up suspended solids and nutrients from the soil, but can also deposit its salts on land, decreasing the conductivity of the water ⁴⁵.

Dams and river diversions affect conductivity by reducing the natural volume of water flow to an area. When this flow is diverted, the effect of additional freshwater (lowering conductivity) is minimized ²³. Areas downstream of a dam or a river diversion will have an altered conductivity value due to the lessened inflow ²³.

 

Conductivity and Water Level

The conductivity of water due to water level fluctuations is often directly connected to water flow. Conductivity and salinity fluctuations due to water level changes are most noticeable in estuaries. As tides rise, saltwater from the ocean is pushed into an estuary, raising salinity and conductivity values ²⁹. When the tide falls, the saltwater is pulled back toward the ocean, lowering conductivity and salinity ²⁹.

Evaporation can cause salinity concentrations to rise. As the water level lowers, the ions present become concentrated, contributing to higher conductivity levels ³⁴. This is why conductivity and salinity values often increase in summer due to lower flow volume and evaporation ²¹. On the other side of the scale, rain can increase water volume and level, lowering conductivity ²⁹.

 

Salinity and Stratification

Temperature and salinity levels alter water density, and thus contribute to water column stratification ²¹. Just as a decrease in temperature increases water density, an increase in salinity will produce the same result. In fact, the change in water density due to a salinity increase of 1 PSU is equivalent to the density change due to a 4°C decrease in temperature ²⁸.

Stratification can be vertical through the water column (seen in lakes and oceans), or horizontal, as seen in some estuaries ⁸. These strata are separated by an boundary known as a halocline ⁹. The halocline divides layers of water with different salinity levels ⁹. When salinity levels differ by a great amount (often due to a particularly fresh or saline inflow), a halocline develops ²⁸. A halocline often coincides with a thermocline (temperature boundary) and a pycnocline (density boundary) (²⁸. These clines mark the depth at which water properties (such as salinity, temperature and density) undergo a sharp change.

Estuaries are unique in that they can have horizontal or vertical haloclines. Vertical haloclines are present when salinity levels decrease as the water moves into the estuary from the open ocean ⁸. Vertical haloclines often occur when tides are strong enough to mix the water column vertically for a uniform salinity, but levels differ between the freshwater and oceanic sides of the estuary ⁸.

Horizontal stratification is present in estuaries where tides are weak. The incoming freshwater from rivers can then float over the denser seawater and little mixing occurs ²³. Horizontal stratification also exists in the open ocean due to salinity and temperature gradients.

Haloclines develop in lakes that do not experience a complete turnover. These lakes are called meromictic lakes and do not mix completely from top to bottom ⁴. Instead, they have lower strata known as a monimolimnion. The monimolimnion remains isolated from the rest of the water column (mixolimnion) due to the halocline ⁴. Meromictic lakes can develop when a saline inflow (natural or man-made) enters a freshwater lake, or if a saline lake receives a freshwater inflow ⁴. (stratification)

As saline water cannot hold as much dissolved oxygen as freshwater, stratification due to haloclines can contribute to hypoxic and anoxic conditions at the bottom of a body of water ²¹.

 

Typical Conductivity and Salinity Levels

While freshwater sources have a low conductivity and seawater has a high conductivity, there is no set standard for the conductivity of water. Instead, some organizations and regions have set limits on total dissolved solids for bodies of water ^14,37. This is because conductivity and salinity can differ not only between oceans and freshwater, but even between neighboring streams. If the surrounding geology is different enough, or if one source has a separate inflow, conductivity values of neighboring water bodies will not be the same.

Despite the lack of standards and the effects of the surrounding environment on conductivity, there are approximate values that can be expected based on source ^13,14:

Freshwater has a wide conductivity range due to geology effects. Freshwater that runs through granite bedrock will have a very low conductivity value ³⁴. Clay and limestone soils can contribute to higher conductivity values in freshwater ³⁴. Some saline lakes exist due to a restricted outflow ⁴. The conductivity of these lakes is dependent on the specific ionic composition present ⁴.

The conductivity of estuaries tends to be the most variable as they are constantly influenced by freshwater and saltwater flows. The conductivity of seawater is dependent on the salinity and temperature of the water ³⁸. Measurements will vary between the Equator and the poles as well as with depth due to conductivity’s dependence on temperature ³⁸.

As with conductivity, the expected salinity of a body of water can only be estimated. Ocean salinity values can vary between 30 and 37 PSU ²². Despite the differences in salinity, the ionic composition of seawater remains remarkably constant across the globe ³. The surface salinity of the ocean is dependent on rainfall. In areas around the equator and coast where rainfall is high, surface salinity values are lower than average ²⁸. These different salinity values contribute to ocean circulation and global climate cycles ³¹.

The following chart offers approximate salinity values in ppt (parts per thousand) ²⁷:

Once a history of conductivity measurements has been conducted, it is easy to see an established range for a particular body of water ¹. This range can be used as a baseline to evaluate measurements as expected (and unexpected) values ¹.

 

Deionized Water

It is important to point out that just because deionized water or ultra-pure water has no extraneous ions, that does not mean that it has a conductivity of 0 uS/cm ⁴⁵. The conductivity value will be very small, and in most situations, negligible, but even deionized water has H+ and OH- ions present. At room temperature, the concentration of both H+ ions and OH- ions is 10⁻⁷ M (think pH – deionized water will have a neutral pH of 7 without atmospheric contact) creating a very small conductivity value ⁴⁶. Despite this low conductivity value, deionized water will still have a salinity of zero; there are no salt ions present, only H+ and OH-, which naturally exist in pure water.

As long as it has not had any contact with air (particularly CO2), deionized water should have a conductivity of 0.055 uS/cm, or a resistivity of 18 megohms at 25 °C ^5,47. If the deionized water has equilibrated with air, the conductivity will be closer to 1 uS/cm (1 megohm) at 25 °C (and it will have a pH of 5.56). Most standards allow for a conductivity range of 0.5-3 uS/cm at 25 °C for distilled water, depending on the length of time it has been exposed to air ^13,14.

Temperature changes will have a greater effect on the conductivity of deionized water (or any nearly pure water), due to the molar equivalent conductivity of H+ and OH- in the absence of other ions ³. Instead of increasing conductivity by 2-3% per degree Celsius, it may increase by approximately 5% per degree Celsius ³.

 

Consequences of Unusual Levels

Unusual conductivity and salinity levels are usually indicative of pollution ¹. In some cases, such as excessive rainfall or drought, they can be connected to extreme natural causes. Regardless of whether the result was caused by manmade or natural sources, changes in conductivity, salinity and TDS can have an impact on aquatic life and water quality.

Most aquatic species have adapted to specific salinity levels ⁴. Salinity values outside of a normal range can result in fish kills due to changes in dissolved oxygen concentrations, osmosis regulation and TDS toxicity ^4,21,37.

When conductivity and salinity values extend too far from their usual range, it can be detrimental to the aquatic life residing in a body of water. This is why fewer, but perhaps hardier, species have adapted to life in estuaries, where salinity is constantly in flux. Estuarine life can tolerate rapidly changing salinity levels better than both their freshwater and marine counterparts ⁴. But even these brackish-water species can suffer if the salinity changes become too extreme.

Cite This Work

Fondriest Environmental, Inc. “Conductivity, Salinity and Total Dissolved Solids.” Fundamentals of Environmental Measurements. 3 Mar 2014. Web. < https://www.fondriest.com/environmental-measurements/parameters/water-quality/conductivity-salinity-tds/ >.

Additional Information

• Conductivity Measurement Methods
• Conductivity Electrodes
• Conductivity Meters
• Applications
• References