Maintaining Water Quality Control Systems

Physical technical water quality control systems are not used as such in crop production, as water quality is not monitored and corrected constantly. Regular testing is done, and corrective action is taken when required. The water quality control system (WQCS) that is discussed in this chapter refers to the system that is put in place to ensure that such testing and corrective testing is done correctly and timeously.

The basic water quality control system (WQCS) involves the monitoring of received water and water leaving the farm. Monitoring must be done at regular intervals, with the frequency depending on the magnitude of fluctuations in the quality over time. The water quality of the input water should however be measured at least twice per annum, once at the beginning of the rain season and once after the rain season. Without these measurements, no improvement or maintenance program is possible.

Another important component of a WQCS is measuring the quality of the water leaving the farm. This is not only done where natural rivers and streams are used to remove excess or drainage water from the farm or orchards but also includes drainage furrows and water lanes. Monitoring and measuring of this nature include analyses of subsoil water to monitor the leaching of nitrates and chlorides.

Maintaining WQCS involves the following:

• Main water source or sources.
• Effluents from rinsing the irrigation pipes.
• Effluents from backwashing the filters.
• Other signs of water contamination.
• Composition of the water leaving the farm.

Main Water Source Or Sources

• Sample all water sources utilised on the farm for irrigation of the trees or for applying foliar sprays at least twice per annum;
• Record all analytical data on a chart or graph.

Effluents From Rinsing The Irrigation Pipes

This information is important to identify the causes of blockages, and for evaluating the possibility of them occurring. Blockages are caused by:

• Salts that dissolve in diluted acid;
• Material that will not dissolve in diluted acid, including sand from the filters, silt and clay, organic debris, and plastic from cutting the pipes during repairs;
• Slime and filamentous material of biological origin.

It is not easy to measure these factors accurately and the quality manager has to rely on subjective observations. Nevertheless, these observations supply useful information.

The most appropriate method is to collect the material, let it dry out to form a paste and then proceed as follows:

• Drop diluted hydrochloric acid on some of the paste. If it dissolves, the paste consists of salts. If it fizzes, the paste contains carbonates. Do not handle the paste after hydrochloric acid has been added to it;
• Examine the deposit for pieces of plastic, seeds, etc.
• Feel the paste between the fingers. A silky feeling indicates clay and silt, while a slippery, slimy feeling indicates microbial deposits.

Click here to view and download a handout that explains maintaining a drip irrigation system for perennial horticulture.

Effluents from Backwashing The Filters

Inspection of these materials will help to identify a change in the quality of the water. An increase in the concentration of clay and silt will indicate that the water source carries more solids. This can be related to the season or current climate but indicates the need for more frequent backwashing.

To inspect the backwash from filters, collect the backwash water in a 50-litre drum. Let all the insoluble material settle, decant the water carefully and inspect the deposits visually and by touch.

Other Signs Of Water Contamination

Contamination is not easily detected except through specific testing as described above. Monitoring fauna and flora, and activities upstream will assist in detecting possible contamination.

Composition Of The Water Leaving The Farm

The Department of Water Affairs (DWAF) requires that the quality of water leaving the farm is similar to the quality of water received on the farm. This means that farm management is responsible to maintain the quality of the water passed onto the next user.