MATERIAL

4.2.4.1 Monitoring of Climatic Factors (Rainfall and Evapotranspiration)

The field experiment showed that the measurement of rainfall and ET was practicable and feasible. Results can be found in Chapter 3. The ET data was not measured at the site but was estimated from the pan evaporation data collected from a nearby site. This appeared to be a feasible and satisfactory alternative to the collection of ET data at the site itself. The rainfall often controls the maximum amount of effluent that can be applied without overloading the hydrological capacity of the soil or reducing the effluent renovation processes. Also rainfall is partially responsible for the hydrological characteristics of a LTS, and thus influences the degree to which many soil and soil-plant processes occur.

Measurements of the rainfall, ET data and the applied irrigation volume (being the input parameters of the water balance) were necessary in order to estimate the drainage losses at the site under different water and crop regimes. At this site the high concentrations of NO3-N in groundwater appeared to coincide with the occurrence of drainage.

4.2.4.2 Monitoring of Soil Factors

Soil moisture level: Monitoring of SMC was important to know how the soil hydrological conditions change with effluent irrigation and rainfall events. Monitoring of SMC was also important to determine the soil moisture storage which was an input parameter to the water balance calculation. The study showed that monitoring of SMC using TDR was practicable and feasible at the site.

Soil nutrient level: Measuring the concentration of soil nutrients (N, P) before and at the conclusion of the experiment was important to enable a nutrient balance to be

estimated, and to provide information on the renovation processes taking place in the soil (data not presented here). However, field experience of measuring the soil nutrient levels showed that it was not practicable and feasible to measure soil nutrient levels at the site on a regular basis as the large number of samples required to counter soil variability, and the stony nature of the soil made soil sampling a laborious and time consuming job.

Soil pH, temperature, bulk density, and infiltration capacity: Field experience of monitoring soil pH, temperature, bulk density and infiltration capacity showed that monitoring soil temperature on a daily basis was very practicable and feasible, but that although it was very important to monitor changes in bulk density and infiltration capacity over time, it would not be feasible to monitor these more than 2 or 3 times a year. This is because of the large number of samples required to counter soil variability and the time-consuming nature of the sampling procedure. Soil bulk density is very important because it reflects the soil structure and the pore size distribution. The pore size distribution determines the soil moisture characteristics, or moisture release curve (Gradwell and Birrell, 1979). Bulk density was measured before and at the conclusion of the experiment.

4.2.4.3 Monitoring of Water Quality

Monitoring of water quality generally involves the assessment of the quality of waters (effluent and natural) entering, within, and leaving a LTS (McMahon and Thorn, 1990).

Effluent monitoring: Effluent monitoring was done to record what was being applied and how nutrient concentrations fluctuated seasonally. The field study showed that effluent samples could easily be taken and sent to the laboratory for chemical analysis. The study demonstrates that monitoring the effluent quality is important in determining the nutrient loading rates and the seasonal variation of nutrient concentrations throughout the experiment.

Soil water: Several methods are available for measuring the movement of water and solutes in the soil profile. The detail of these methods can be found in Burt et al.

(1993). Addiscott (1990) have reviewed the different techniques. In general there is no single preferred technique, as each method involves different degrees of effort, and each is, to some extent, unsatisfactory. The choice of any one method therefore remains to a degree a compromise between what is ideal and what is practical. In this study, porous ceramic cup samplers were used to extract water from the soil by applying suction, so that solute concentrations in the soil water could be analysed using normal laboratory methods. However, in the field experiment (Chapter 3) the difficulty in obtaining sufficient numbers of soil solution samples cast doubts on the suitability of this technique for routine monitoring at a LTS. There may also be problems with the use of porous ceramic cups (Addiscott, 1990), that are related to the nature of the soil pores that are sampled and the degree to which the water sampled is representative of all the water in the soil and from management point of view, it is too late to take any effective management action by the time the information regarding the nutrient concentration level in the soil water samples is received.

Groundwater: Monitoring of groundwater beneath effluent irrigation sites is an essential indicator of environmental performance (Bond et al., 1998). The field experiment showed that the groundwater samples can easily be taken and sent to the laboratory for the chemical analysis of nutrients, but once the groundwater is contaminated it is too late to rectify the problem in the short term.

The field scale study demonstrates that the knowledge of groundwater conditions and how these conditions change as a result of climate, SMC, and effluent irrigation is necessary for the effective management of LTS, and confirms that monitoring of groundwater within the vicinity of a LTS may provide a useful means of assessing the impacts of effluent irrigation on the local environment (McMahon and Thorn, 1990). In the study reported in Chapter 3 the effluent irrigation was stopped at the site following the information on groundwater NO3-N concentrations becoming available.

4.2.5 Summary and Conclusions

The rationale for this research was that monitoring of SSF is necessary for the effective control over the impact of leached NO3-N on groundwater quality. Information received from the analysis and interpretation of SMC, rainfall, and groundwater quality data was helpful in understanding the distribution and movement of N through the soil-water matrix into groundwater at the site. The results of the study showed that short term management decisions can be made by monitoring the capacity of the soil to absorb the designed hydraulic loading rate (by calculating the difference between the SMC of a particular day and the field capacity of the soil) and the possibility of rainfall occurring in the immediate future. The study showed that when the SMC approached a level which was close to or at field capacity, further rainfall caused NO3-N leakage to occur and resulted in the MPL of 11.3 mg/L being reached or exceeded in the shallow groundwater at this site.

Monitoring the ability of soil to absorb the designed irrigation volume and likely rainfall in the immediate future is a cost-effective approach to estimate the potential for NO3-N leakage through the soil-water matrix into groundwater. The approach is cost effective in the sense that little time is required to regularly monitor the SMC, and it is easy to check the possibility of rainfall in the near future. If this is done, continuous monitoring of groundwater may not be required, and money can be saved from the laboratory analysis of groundwater samples. Groundwater samples can be taken three or four times a year as a check.

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CHAPTER 5

LABORATORY SCALE APPLICATION OF THE DECISION SUPPORT