School of Environmental Science
Division of Science and Engineering
Murdoch University in Perth, Western Australia
Adjunct Associate Professor
Originally Prepared for the Buddhist Society of Western Australia (Inc)
21 January 1998
Dr. Bill Scott, Mr Theo Bazen, Mr. John Rich and Mr. David Williamson visited the property owned by the Bodhinyana Buddhist Monastery on 21 January 1998. We were hosted by Abbot Ajahn Nyanadhammo and Mrs Sue Towler.
The purposes of the visit were:
The location is shown on the attached map (Figure 1). The area is estimated to be about 1 km2 (100 ha). By driving along Kingsbury Drive it was found that the catchment was primarily a logged forest area with a landing ground in the upper reaches, 40 metre wide and about 600 metre long, the only area of cleared land. CALM maps (prepared in 1973) show gravel pits located north and south of the western end of the landing ground, but field inspection suggests that these were rehabilitated with tree plantings over 10 years ago. It was confirmed that the area of the catchment east and upslope of the monastery site is Crown land managed by CALM. The catchment is a narrow one with quite step slopes, typical lateritic soils and frequent exposures of granite rock. The lower boundary of the monastery property is virtually on the escarpment of the Darling Scarp.
The Top Dam (see I on Figure 1 and picture below) existed when the property was acquired for the monastery in 1983. The Middle Dam (II on Figure 1), downslope but adjacent to Top Dam, was constructed by the Monastery in April 1993 to receive the winter overflow from the Top Dam. It has been observed that the water level in the Top Dam does not appear to decline as might be expected as water is removed.
Bottom Dam is a new dam built in June/July 1997 about 200 metres down the valley from Top Dam and Middle Dam. Within a fortnight of completion of construction the dam filled to overflowing. The site has a vertical granite face on the northern side into which the earthen wall of the dam is butted. As would be expected there is seepage coming from the interface of the dam wall and the granite face. On the eastern side of the dam there is another vertical granite face above the level of the dam where flow in the creek forms a waterfall into the dam. Problems with scouring during overflow of the dam are being attended to, and further earthworks undertaken to correct the problem of inaccurate levels at the top of the dam wall. It was recommended that a concrete and rock spillway be designed and constructed to handle the overflow from this dam and to avoid the potential damage to the dam wall by scouring during overflow. Professional engineering advice should be sought on this matter to avoid a catastrophe.
The available data for salt content of the water in the dams showed the quality to be marginal to brackish for all dams depending on the time of the year. See the analysis of water from the dams (appended in a table at the end). Presumably, if analyses were available for the water quality following refilling of the dams during winter, the quality at the end of the winter would be in the potable range (that is, less than 500 mg/L total soluble salts or an electrical conductivity of less than 850 micro Siemens per centimetre (μS/cm) depending on the units used). Water used from the dams is aerated in a tank before being pumped to storage tanks to reduce the high iron content. The water is classed as hard with the potential to cause corrosion problems in pipes, hot water systems and appliances.
As the salinity issue will be inevitably linked to groundwater flow into the dams and groundwater discharge at the soil surface, indications of seepage were sought. Signs of seepage into Top Dam were found immediately up-slope of the present water level in the dam, with areas of salt inflorescence observed (see Salt Scald below). Reeds, usually associated with areas of groundwater discharge, were also present. There was an exposure of granite rock associated with the wall of Top Dam suggesting that there could be a granite rock bar across the valley which would act as a barrier to groundwater flow in the valley at the location of Top Dam. This would explain the groundwater discharge at the surface at the dam location. Middle Dam was located on an area known to be a wet seepage area. To the south-east of the dam an area of moist dark coloured soil was observed which identified an area of groundwater seepage. There was no information available, and no wells or dug holes available, to determine the salinity of the groundwater in any of the observed seepage locations.
4.1 The salinity level in each dam is the consequence of the dam receiving discharge of saline groundwater. The source of the groundwater would be from aquifers within the topographical catchment for the dams.
4.2 The catchment for the dams is degraded due to the historic logging of timber, but the vegetation present throughout the catchment is adequate to avoid the classic scenario for salinisation where groundwater levels rise due to increased recharge caused by land clearing. Even in this high rainfall location (estimated to be about 1100 mm per year) groundwaters can contain high salt content associated with the restriction to natural groundwater flow caused by geological barriers cutting across the valleys in catchments. There is good evidence of both the barriers and the impact on groundwater flow (as seen in the seepage zones) in the catchment where the dams are located.
The high rainfall in this location provides the potential for a reasonable quantity of groundwater recharge to occur in these lateritic soils which could sustain the wet areas observed even under the present or previous forested condition of the catchment. From data presented by Stokes et. al. (1980)1, the salinity of the groundwater could be in the range of 1000 to 2500 mg/L total soluble salts, and the total quantity be in the range of 100 to 200 tonnes per hectare. An estimate for annual discharge of salt from the catchment is of salt stored in the catchment could about 50 kg/hectare, though this may have been increased to as high as 100 kg/hectare due to the effect of logging for millable timber and an associated increase in the discharge of groundwater from the catchment.
4.3 The observation of the fairly constant water level in Top Dam suggests that there is a significant volume of groundwater seeping into the dam. This would carry salt into the dam and allow an increase in salinity following the refilling of the dam with runoff water during the winter months. Evaporation would contribute to the increase in salinity, but with a smaller impact than the seepage of saline groundwater into the dam.
4.4 All 3 dams are subject to salt input via the groundwater seepage, though there may be indirect flow into Middle Dam. To predict the future trend of salt input to the dams and, consequently, the trend in the salinity of the water in each dam, would require a detailed study of the hydrological system which maintains the water resource in the dams. This requires quantifying the input and output components for both salt and water. Further comment on this follows.
4.5 There is no perceived advantage in supplementing the existing vegetation in the catchment to try to reduce recharge to the groundwater as is the objective for managing salinity in the agricultural regions. The area of the landing strip is less than 5% of the total catchment area and would not have significance through any additional recharge produced in the overall situation of the mobilisation of salt in the catchment. There is no rational or economic means of determining where there may be significant enhancement of recharge in the catchment which might be modified by intensified tree planting or by engineering methods. Nevertheless it is reasonable to seek to maintain the existing vegetation, though additional plantings could only be justified for aesthetics rather than recharge management.
The existing management of water requirements is very appropriate. It is understood that, in general, personal water needs are obtained through rainwater tanks and other needs are met from the dam supply. The cleaning out of gutters and the periodic cleaning of rainwater tanks is carried out, and this is highly commended for managing quality of rainwater.
Other options which could be considered include:
Preventing or reducing the groundwater seepage into the dams is a logical consideration. The constraints are primarily ecological and economic.
The opportunity for a scientific project would be focussed on the prediction of the future salinity of the water in the dams. Fundamentally, this would be a water and salt balance study to produce a model of the hydrological system. The study would aim to provide a prediction of the salinity trend for each dam and establish a working model which could be used by the Monastery to continue monitoring the situation.
The project would:
A conceptual diagram of the field system is given in Figure 2, below.
Resources would be needed to make the necessary observations and installations at the Monastery site. These could include installation of groundwater wells/piezometers, water level gauges (and possibly data loggers) for dam water and groundwater level monitoring, raingauges, analyses of water samples for salinity and microbiology, and surveying to determine the shape of the dams. An estimate of evaporation from the dams would be required, probably using meteorological data available from the Meteorological Station at Karnet.
We acknowledge with thanks the interest and hospitality of the Monastery in involving Murdoch University in this problem and for providing copies of the information already obtained in previous investigations by various agencies. We are encouraged by the interest in supporting a student project.
Converted to html 20Dec03 from original report prepared 23 January 1998 (B. Scott)
File: MonastryDamSalt1998.doc (D.R. Williamson)
Australian Environmental Laboratories
Client: Bodhinyana Monastery
Our Reference: 37573
Project: Dam Waters
Type of Sample
|Total Dissolved Solids (calc)||mg/L||PEI-032||1500||960||830|
|Iron, Fe (soluble)||mg/L||PEI-001||<0.05||<0.05||0.10|
|Sum of ions||mg/L||Unassigned||1530||766||720|
|Cations/Anions % difference||Unassigned||-0.57||-3.42||-0.57|
1 Stokes, R.A., Stone, K.A,. and Loh, I.C. (1980) Summary of soil salt storage characteristics in the northern Darling Range. Water Resources Branch, Public Works Department W.A., Technical Report No. WRB 94.