The importance of monitoring groundwater (watertables) and salinity
The importance of monitoring groundwater (watertables) and salinity
Vegetation clearing since European settlement has altered the water balance of many catchments. Deep-rooted perennial vegetation has been replaced with shallow-rooted annual species that do not use water to the same extent, resulting in more water seeping through to the groundwater. Annual pastures and crops do not use more than 400mm / year of water, and often much less. As a consequence, over time watertables can rise and concentrate dissolved salt within the rootzone of pastures, crops and trees. In areas at risk of salinity it is important to monitor the levels of groundwater or watertable heights and the salt concentration of this water.
Watertables form when water cannot move into or pass through a less permeable layer (e.g rock, clay or other hard layer). Watertables can be very localised and confined to small areas of a catchment, or can be a part of a larger groundwater system covering many square kilometres. It is important to speak to local experts – such as Salinity Extension Officers or Catchment Management Authority staff to understand if your farm is likely to be above a localised, intermediate or regional groundwater system.
Why monitor watertable height and salinity concentration?
Monitoring the watertable heights and salinity of the water you use for agricultural and domestic purposes will allow you to:
- evaluate whether there is a threat of rising groundwater and salinity risk
- determine whether there is groundwater salinity problem on your farm and if so, to define its trend,
- understand how the groundwater system responds to wet and dry years,
- participate in catchment action group activities such as groundwater data collection and catchment planning for salinity management, and
- work out what is the appropriate management action.
Groundwater (watertable) levels are measured using bores, of which there are two types:
- Piezometer: Piezometers consist of sealed PVC pipe of 20 – 40 mm diameter with narrow slots cut in the bottom 25 cm to allow water entry. A piezometer allows you to assess whether there is a saturated zone (watertable) present. Shallow piezometers (for example just above a clay subsoil layer) can help assess whether there is a waterlogging issue. A non-pumping, deep (greater than 3 m deep) piezometer can be used to measure groundwater pressure.
- Testwell: A non-pumping, shallow bore (often less than 3 m deep) made of 40 mm P.V.C. pipe, allowing access to ground water so that its depth and salinity can be evaluated.
Current costs of installing a 50 mm diameter bore are about $100/metre. Your local Salinity Extension Officer will be able to advise you as to suitable contractors, and/or if he/she thinks it is worthwhile for you to install a bore.
Why worry about rising watertables?
Rising groundwater (or watertable) is a major indicator of the risk salinity. Once the watertable rises to within 2 metres of the soil surface there is large risk of salinity developing. Whilst not all groundwater is saline, even relatively low salinity groundwater (eg <500 to 800 uS/cm) can develop into a saline discharge site. Monitoring watertables will allow you to work out whether there is a risk of salinity. Lowering the watertable is the first step to effectively reclaim a saline site.
What salt are we talking about?
When we think of salt, we usually think of table salt or sodium chloride, but there are many other compounds that are also known as salts. These include chloride salts of calcium, magnesium and potassium, as well as bicarbonates and sulphates of sodium, calcium, magnesium and potassium.
Sodium chloride however is the most common salt found in the surface and underground water bodies (called groundwater systems) in southern Australia. Table 1 outlines the salinity concentrations measured in 2 commonly reported units.
Why worry about saline water tables?
The sum total of all soluble salts determines the suitability of water for domestic, garden and agricultural use. Groundwater salinity may impact in various ways:
a) On human health
Saline water becomes unusable not only because of the bad taste but because it affects human health (eg. kidney function is affected by excessive salt intake).
b) On domestic, industrial and community infrastructure
- As salt levels increase in water, it becomes 'harder', affecting its suitability for domestic use.
- Pumps, metallic pipes and tanks corrode. Corrosion of copper pipes lead to high levels of dissolved copper in drinking water, which is harmful to humans.
- Salt will also affect the function of devices like air-conditioners, batteries, boilers, engine radiators, swimming pools etc.
- Salinity also leads to large costs for communities, such as damage to roads.
c) On livestock
Salt affects the health and productivity of livestock, especially pregnant females. Table 2 outlines the salinity tolerance for livestock drinking water.
d) On soils and plants
- High soil sodium concentration (sodicity) leads to soil structure decline through the breakdown of clay particles. Wind and water more easily erode a soil affected by salinity.
- Salt affects the growth and development of crops and plants, especially seedling emergence and early growth. Salt influences how easily plants can extract water from the soil. Plants either store salt within their cells or try to exclude it. Either way, plant growth and function is reduced, and can ultimately lead to death of the plant. Table 3 outlines the salinity thresholds for plant types
Monitoring watertable rise and salinity concentration
STEP 1. Planning where you are going to sample:
Contact your local Salinity Extension Officer (Department of Primary Industries or local Catchment Management Authority) to work out if you are in a salinity prone area and if so, whether and where it could be useful to install a bore or bores on your property.
To monitor watertable height and salinity concentration, the following steps are suggested:
STEP 2. Getting organised:
- A fox whistle or electronic unit attached to a tape measure for measuring the water table (refer to the photo).
- A water sampler, or bailer: a bailer can be made from PVC or stainless steel pipe and has a ball valve at the base which closes and seals the water column upon bailer retrieval (refer to the photo).
- A clean plastic container to hold the water sample.
- A conductivity meter - also called a Total Dissolved Salts (TDS) meter.
There are several models available for field tests, similar to the device you can see in the picture below and all work on the same principle. The two probes (or electrodes) situated at the bottom of the meter measure the conductivity of the water that reflects the amount of dissolved salts present in the sample (refer to the photo).
- Recording sheets.
Fox Whistle Unit
A Conductivity Meter
(Fox Whistle Unit and Bailer photographs from: Centre for Land Protection and Resources publication – "Groundwater Monitoring - How and Why?")
STEP 3. Planning when to monitor:
Water table and water salinity trends need to be monitored over several years. The frequency of monitoring the watertable will be determined by how responsive it is to seasonal conditions and rainfall events and also the duration that this monitoring has already been completed. Initially the monitoring interval could be monthly but may then be extended to a three to six month basis after one or two years.
- For groundwater that is saline, you should monitor the salinity status about 4 times/year, suggested for ease of remembering to be January 1, April 1, July 1, October 1.
STEP 4. Recording the watertable height and sampling the water salinity:
- When you are ready for sampling, have a talk to your Salinity Extension Officer about sampling issues and make sure you are consistent once you start monitoring. Factors such as whether you take the water samples from the top, middle or bottom of the water column in the bore can affect the water salinity results.
- Firstly, measure the watertable height. This can be done using either a simple fox whistle unit or a more elaborate and expensive electronic unit. Both types of units will produce an audible whistle once the end of the unit strikes the surface of the watertable. Measure the watertable height reading to the top of the bore casing. Then from this reading subtract the distance from the top of the bore casing to the natural soil surface at the bore site. This final reading will be the watertable height from natural soil surface.
- Secondly, measure the salinity status of the water by lowering a bailer into the bore below the water surface.
- Remove the bailer, pour out the water, and repeat the exercise. Then collect the water sample into your clear plastic container for salinity monitoring.
Most people who test their own bore water salinity also measure the ground water depth at the same time.
STEP 5. Measure the salinity of the sample:
- Take the cap off the Conductivity Meter.
- Press the on/off button to turn it on and hold the electrodes of the meter in the water. Do not totally submerge the meter in the water, or it may be damaged.
- Move the base of the meter so that water swirls around the electrodes.
- When the reading stabilises or continually jumps between two numbers, press the 'hold' button.
- Read the result: in EC or mg/L.
- Wash the electrodes of the EC meter in clean non-saline water before storing the instrument.
It is important to understand that your instrument does not give you any information about the type of salts that may be in your sample, just the total amount of dissolved salts.
Note regarding calibration of your conductivity meter:
Calibration means reading a solution of known conductivity and adjusting your meter to read the same value. Your conductivity meter needs to be calibrated on a regular basis: every three months is usually recommended. Use a standard calibration solution that is within the range of your EC meter. To calibrate the meter:
- Pour a small amount of the solution into a small clean container
- Turn the meter on and place the electrodes in the solution, swirling it gently
- Read the meter once the reading has stabilised
- If the meter is not reading the same value as the calibration standard then the meter needs to be calibrated. Provided that your calibration solution is tightly sealed, kept from extreme of temperatures and out of the light, it will last for at least one year.
- If you have problems in calibrating the meter, seek expert advice from your local salinity extension officer
STEP 6. Record the results
Keep a record of your results in a place where you can find them again and compare with subsequent results.
STEP 7. Interpreting your results
The number of different methods used to express water salinity concentrations can be confusing. MicroSiemens per centimetre (�S/cm) is the most common unit used for water salinity. It is commonly referred to as EC (electrical conductivity). Another method used to measure water salinity is as Total Dissolved Salts (TDS) with mg/l or ppm being the most common units.
The following table shows how to convert a salinity reading into different units:
Table 1: Comparison of how salinity is measured
|�S/cm (EC)||mg/l or ppm (TDS)|
Conversion formula: 100 uS/cm (EC) = 64 mg/l (TDS)
Salinity threshold for human consumption
The maximum concentration of salt in water that is deemed safe for human consumption is 830 EC, which corresponds to about half a gram of salt per litre of water.
Salinity thresholds for livestock
Table 2: Salinity thresholds for livestock drinking water
|Maximum concentration for reasonable growth rates uS/cm(EC)||Maximum concentration at which livestock can only be expected to maintain their condition uS/cm (EC)|
|Sheep, dry feed||10000||22000|
Note that livestock vary in their ability to tolerate salt in drinking water depending on the pasture composition, their age, condition, reproductive status and the weather. Other factors may also affect water quality such as contamination with heavy metals and trace elements at toxic levels.
Salinity thresholds for some plants and crops
Table 3: Salinity thresholds of water used to irrigate the following plants and crops
|Plants and crops||Upper limits of water salinity|
|uS/cm (EC)||mg/l (TDS)|
What to do with my water salinity data?
Your monitoring data will be part of a much larger story on what is happening with salinity in your area. Discuss with your local Salinity Extension Officer how to make best use of the data you are collecting and how it can be linked in with data others are also collecting.
- If you have problems interpreting your results, seek help from the local office of the government agency (e.g. Agriculture Victoria) that deals with salinity or water quality.
- Find out how your level of salinity will impact on your production.
- Find out about what the Best Management Practices are for salinity on your farm (refer to Table 4 for a start) and seek expert local advice.
- Review your farming system to see where you can improve your water use efficiency.
- Develop a whole farm plan to tackle your farm's environmental challenges in a strategic manner.
Table 4: Best Management Practices for salinity
|What do we need to do to manage salinity?||Actions to reduce salinity|
|1. Protect and manage our native vegetation.||* Remnant areas should be fenced and managed to promote species diversity and seedling recruitment.|
|2. Change the mix of vegetation in recharge areas so that less water leaks through to the groundwater.||* Incorporate deep-rooted perennial plants, such as lucerne, phalaris, cocksfoot and trees into your farming system.|
|3. Use water more effectively and efficiently.||* Improve water use efficiency of crops by paying attention to crop selection, timely sowing, good crop nutrition and disease and weed control and by intercropping lucerne stands.|
|4. Make better use of land affected by salt.||* Use of salt-tolerant pastures eg. Tall Wheat Grass, Puccinellia in discharge areas. * Revegetation for wood products and possibly carbon credits may be a viable option.|
Breaches of the Water Act
Breaches of the Water Act may occur if appropriate approval has not been sought from the local Water Authority when soil sampling, installing piezometers or neutron probe access tubes. In general, if any soil sample, piezometer, soil moisture tube, is greater than 3 metres deep, or if it intercepts the groundwater, it may have to be registered with the relevant water authority. Individual water authorities could have different approaches to registration. Breaches of the Water Act are an offence that involves a financial penalty.
Registration requires a series of steps that generally involve:
- contacting the local water authority and applying for installation licence
- this licence will require maps of locations, depth, purpose and information regarding near by infrastructure. A dial before you dig, that locates underground infrastructure, power etc may also be required. If the person has never registered a bore, or a hole in the ground so to speak, then it is strongly advised they contact the water authority to determine the steps involved.
- the cost to register - roughly $400 for first hole and an additional $50 for second etc, but this may vary with the water authority.
The registration process exists for several reasons. For example:
- in an attempt to prevent pollution by aquifer leakage, this is why soil sampling is included in the registration process if groundwater is intercepted.
- to provide an identification number and location of groundwater bores, and generate funds to run the groundwater data bases.
- to ensure no illegal groundwater extraction.
The registration process also means that only a registered driller can install the holes or collect the soil samples. If you unsure of the groundwater depth in your region, then using a registered driller removes any risk.
There are generally no exceptions to the rule, however there are variations in how the Act is governed by water authorities, so contact with water authorities is essential. Also, as a low cost risk management strategy, it is recommended to contact the Dial Before You Dig website or hotline (phone 1100) and ascertain the location of any infrastructure (power, telecommunications, gas etc.).