Water quality: Assessing fitness for agricultural purpose usage

DR JAMES MEYER, DEPARTMENT OF ANIMAL AND WILDLIFE SCIENCES, UNIVERSITY OF PRETORIA

Agricultural water use is recognised in the 1996 Second Edition of the South African Water Quality Guidelines, published by the then Department of Water Affairs and Forestry, as having three main components, namely Irrigation (Volume 4), Livestock Watering (Volume 5) and Aquaculture (Volume 6).

Whilst this is a convenient classification, in reality other recognised water uses are often also applicable, specifically for large scale agricultural enterprises. This includes water utilised by staff (e.g. drinking, food preparation, bathing and laundry), water for disinfection processes (e.g. chlorination and UV applications), industrial processes (e.g. heating and cooling processes) and effluent treatment for discharge compliance.

The focus on water for agricultural uses is typically on volume with the consistency of supply and costs associated key issues. However, with the recognition of water scarcity and increasing emphasis on water resource management, with various licences and permits applicable, it is increasingly argued that the management of water quality should receive similar attention.

The fundamental point of departure is that in order to manage water appropriately, the quality thereof must be measured, monitored and allocated accordingly to various uses by either matching the inherent quality to the most suitable use, or alternatively by treating the water to the required quality for the intended use.

In order to accomplish this, “fitness for use” is largely defined by the use of water quality guidelines. Thus, use of water for agricultural purposes may require the application of many different sets of water quality guidelines.

Background

As an example of the concepts of a water quality guideline and different possible uses, most people can relate to the concentration of fluoride in their drinking water.

Whilst the causative role of excessive fluoride (a water quality constituent or WQC) and adverse health effects such as dental staining and pitting (enamel hypoplasia) and even skeletal abnormalities are well documented and described, it is also recognised that fluoride has essential functions in the body, prompting some highly publicised attempts previously to fluoridate drinking water.

In order to arrive at a position to understand the management options applicable, a few fundamentals have to be met. The biochemical pathways involving toxicity have to be comprehensively identified and linked to an exposure dose which is a combination of water intake and WQC concentration.

The WQC must also be routinely determined by analytical procedures to be present in order to validate exposure and also at sufficiently sensitive detection limits to accurately determine the concentration. In the case of benefits associated with fluoride ingestion, clear evidence of essentiality is also required.

A guideline for fluoride for domestic use may thus present varying options at different concentrations for different uses, for example (Quality of Domestic Water Supplies: Volume 1: Assessment Guide, 1998):

• Target Water Quality Range (TWQR) for fluoride for drinking water = 0 - 0,7 mg/litre.

This implies that the probability of adverse effects experienced by a wide range of human user types (pregnant women, children, adults, etc.) following exposure (consumption) due to fluoride toxicity is insignificant and highly improbable. Alternatively it may be considered that the risk associated with exposure is acceptable.

At a concentration range of 1,0 mg/litre - 1,5 mg/litre the drinking water guideline description of the probable types of effects looks a bit different:

• Increasing adverse effects in sensitive user groups: 1,0 mg/litre - 1,5 mg/litre.

What this is indicating is that the probability for adverse effects of a significant nature has increased. If the concentration increases to beyond 3,5 mg/litre, then the guideline indicates that the water is completely unacceptable for drinking purposes.

The same statements that describe the “types of effects” and thus give guidance on the “fitness for use” are true for using the water for food preparation and thus do not only apply to drinking water, but extend to another recognised water use. However, across all of these concentration ranges for fluoride the water remains classified as “ideal” for bathing and laundry – also recognised water uses.

The key message is thus that the fluoride may be of critical importance for some domestic water uses, but of little consequence to others. A similar strategy is applied for irrigation and animal watering water quality guidelines.

Here are some central themes for these agricultural water uses:

Irrigation

Water quality guidelines assist to determine the fitness for use by comparing the water quality in the form of WQCs that are present and the concentrations at which they occur. However, the application of these guidelines is not simply to predict the outcome of effects ranging from desirable to undesirable, but also to enable to mitigation of these potential predicted problems.

As an example of considerations for irrigation and the WQC Nitrogen (inorganic) includes:

• Crop yield and quality : TWQR <5 mg/litre.
• Groundwater contamination: TWQR <5 mg/litre.
• Irrigation equipment: TWQR <0,5 mg/litre.

The concern for irrigation equipment in this instance relates to the conditions for the growth of nuisance plants and algal blooms.

When considering the effects of Total Dissolved Solids (TDS) for irrigation, the considerations also include these issues noted for Nitrogen, but for different types of effects.

For most of these norms the primary focus is on crop sensitivity to irrigation water-induced soil salinity and the application to soil and effects on sustainable use of irrigated soil. In addition, the concerns also relate to damage to irrigation equipment by a wide range of effects from corrosive actions to the formation of scale.

For both of these, WQC examples of mitigation options when the concentration exceeds the TWQR, include leaching strategies, irrigation frequency manipulations, altering fertiliser application, selection of appropriate crop types and the treatment of water to protect the irrigation equipment.

Due to increasing concerns for product quality (often strict export quality stipulations), supply (quantity and consistency) and pressure on scarce resources, the appropriate allocation of available water using the correct methods and crops, is receiving more research attention.

Animal uses

Although the previous guidelines refer to “livestock watering” – the recognition of water quality for wildlife, companion animals, research facilities and specific activities, for example various forms of equine events – the term is more accurately described as animal uses.

Norms used to judge the fitness for use include issues relating to health, product quality, environmental aspects and similarly to irrigation concerns for watering equipment.

The environmental aspects are receiving increasing research funding as confined animal feeding operations may impact on environmental conditions ultimately affecting water resources. This extends to game ranching and the provision of water for wildlife where the act of providing water affects game movements and subsequently herbaceous communities and sacrifice zones.

Some issues have more importance than others, depending on the type of production system (commercial intensive or rural subsistence). An example of this may be found in intensive production systems where feed intake is a critical parameter for profitable performance. The correlation between feed and water intake requires added emphasis on the “taste” or aesthetic properties, as water that may be “safe” may still have a poor palatability that can lead to reduced intake and thus growth.

Similar to irrigation operations where water usage is monitored, water intake by animals should also be determined to sufficient accuracy to assess any significant declines that may hint at possible production problems.

In many cases, intensive production systems may also apply a variety of treatment processes to disinfect water or reduce the impact of inherent quality on watering equipment (e.g. de-scaling). It should be borne in mind that the selection of the appropriate disinfection process is dependent on inherent water quality and also inevitably requires that the process is monitored and therefore managed.

Additional water quality aspects relevant to agricultural uses

The source quality may be varied in agricultural water uses with sources potentially containing water-borne pathogens and poisons (e.g. toxic algae). Some water quality assessments, for example microbiological indicator organisms (Total Plate Counts, Faecal Coliforms and E. coli), should be performed at specific points in the water distribution system, depending on the water use.

A key research topic world-wide currently investigates a phenomenon referred to as endocrine disruption and endocrine disrupting compounds (EDCs). The focus is for both compounds related to the application and use thereof in agriculture (for example pesticides) as well as ones that are naturally occurring (for example lead and bromide in groundwater).

Some EDCs may reflect a combination of naturally occurring and inputs or contributions by anthropogenic activity (for example nitrate). Whilst the earlier example used for fluoride and domestic drinking water is well described, when attempting to assess the relevance of WQCs that affect the endocrine system, less clarity exists.

In 1991 a hypothesis was put forward to suggest that many xenobiotic chemicals released into the environment by human activity disrupted wildlife and human endocrine systems at ecologically relevant concentrations. When reviewing EDCs, terms such as “potential mechanisms”, “end-point concerns”, “multidisciplinary”, “bio-assay screening” and “suggested links to” are typically encountered.

Endocrine Disrupting Compounds (sometimes referred to as “chemicals” or “contaminants”) do exactly what their name suggests, namely, they disrupt the endocrine system of the body. This has the potential to implicate major events in the body, from growth and development via the thyroid gland to reproduction via the reproductive organs.

Published findings relating to reproductive health suggest these chemicals may be the primary cause behind increases in breast and reproductive tract cancers and reduced fertility. Adverse effects linked to altered immune function as well as cognitive and neurological development are also well documented. Although reproductive abnormalities are mostly reported, thyroid and immune system dysfunction is currently receiving significant research focus.

Increasingly, the list of EDCs is growing to not just include an enormous list of organic pollutants, but also naturally-occurring inorganic constituents, with nitrate a major current concern. This effectively implicates both non-agricultural and agricultural sectors in the detection of EDCs in water sources.

Main points

A multitude of local and international water quality guidelines are available that can be used to appropriately and responsibly manage and utilise water. Different guidelines may reflect different priorities. For example, at a national level the animal health authorities focus on acute conditions, infectious diseases and veterinary public health. It is recommended that specialist advice should be sought.

However, similar fundamental principles apply. Firstly, the WQC must be routinely detected in the water source for it to be considered relevant to health aspects. This in itself implies analytical competency, adequate sensitivity in terms of detection levels and interference and capacity. Secondly, the claimed adverse effect or relevant end-point must be scientifically linked to the suspected chemical.

The standard sequential approach should be:

• First determine the inherent water quality of the water being used.
• Secondly, assess any changes that may be made to the water by abstraction, storage, delivery and treatment processes.
• Lastly, by using these first two steps, assess the feasibility of manipulating or mitigating the water quality to best suit the intended use. This final refinement stage requires an additional aspect of water quality monitoring.

The Department of Water Affairs is currently reviewing the National Water Quality Guidelines and developing a Risk-based version. The Water Research Commission is also actively funding EDC research topics. These are crucial steps to achieving appropriate applications of relevant legislation regarding water quality, water resources and related environmental issues.

For more information, contact Dr James Meyer at 083 5693 167 or
(012) 420-4018.

Further reading can be downloaded from www.wrc.org.za and www.dwa.goc.za.