Johann LanzSoil Scientist (Pri.Sci.Nat.) Reg. no. 400268/12
Cell: 082 927 9018e-mail: [email protected]
1A Wolfe StreetWynberg7800Cape TownSouth Africa
An assessment ofthe impact on soil quality of using treated effluent for irrigationat the County Fair Primary Processing Plant near Fisantekraal
Report byJohann Lanz
June 2019
Table of Contents
1 Introduction and aim........................................................................................................................12 Methodology....................................................................................................................................13 Results and discussion......................................................................................................................2
3.1 Assessment of effluent water quality.......................................................................................23.2 Field assessment of soils...........................................................................................................23.3 Chemical assessment of soils....................................................................................................5
4 Conclusions.......................................................................................................................................65 References........................................................................................................................................7Appendix 1 – Laboratory Soil Analysis..................................................................................................8
1 Introduction and aim
Johann Lanz was contracted to conduct a study with the aim of assessing the impact on soil qualityof using treated effluent from the County Fair Primary Processing Plant near Fisantekraal forirrigation on the property. The request for such an assessment came from the Department ofWater and Sanitation.
The County Fair Primary Processing Plant produces and treats approximately 3,300m3 of effluentper day, some of which is irrigated onto approximately 70 hectares of kikuyu grass pastures on theproperty. The rest of the effluent is used for irrigation by Groenvlei Grass on the adjacent propertyas well as discharged into the Mosselbank River.
2 Methodology
The assessment was conducted by way of:
1. assessment of the chemistry of the effluent water (from existing records)2. field soil investigation of irrigated areas3. soil chemical analysis of irrigated soils
A field soil investigation was conducted by way of excavated test pits across the irrigated areas. Atotal of 11 test pits were investigated on 6 May 2019 by Johann Lanz. The positions of the test pitsare shown in Figure 1. Soils were classified and assessed in terms of agricultural and irrigationsuitability as well as for any visual evidence of soil quality impacts such as surface sealing.
Composite soil samples were taken from test pits according to Table 1. Separate top (0-30cm) andsub soils (30-70cm) were sampled and analysed for a standard agricultural analysis at Bemlablaboratory in Strand.
Table 1. Identification of soil samples
Soil sample number Test pits
1 1, 2, 3, 4
2 5, 7
3 6, 8
4 9, 10
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3 Results and discussion
3.1 Assessment of effluent water quality
Data from a monthly analysis of the effluent water over a four year period was supplied by CountyFair. This data is summarised in Table 2.
Table 2. Summary of analyses of effluent water over time.Measured parameter Minimum Average MaximumpH 6.55 7.48 8.08Electrical conductivity (mS/m) 103 179 251Suspended solids (mg/L) 2 11 59Chemical oxygen demand (COD) (mg/L) 10 53 121Orthophosphates (P) (mg/L) 0.8 26.2 70.9E.coli 0 1322 2000Ammonia (N) (mg/L) 2.4 67.3 133.0Nitrate (N) (mg/L) 0.2 8.6 42.0
In terms of impact on soil quality, the only parameter of possible concern is the salinity of thewater, as indicated by the electrical conductivity, and the related potential sodicity, which has notbeen specifically analysed. All of the other above parameters will have no impact on soil quality.pH of the water will have little impact on soil pH or other soil parameters. Suspended solids andCOD do not influence soil. P and N will increase the nutrient status of the soil, but will have nosignificant negative impact. E.coli will have no influence on pastures. It can only have a potentialimpact if applied directly to plant parts eaten by humans.
High salinity water has the potential to increase the sodicity of the soil, which can impede soilphysical conditions and pose a salinity threat to crop plants. However the salinity of the water, at amaximum of 251 mS/m, dose not pose a significant salinity threat to kikuyu pastures. According toDepartment of Water Affairs and Forestry (1996), a 90 % relative yield of moderately salt-tolerantcrops can be maintained at an EC of up to 270 mS/m.
The impact on soil sodicity is addressed in section 3.3 below.
3.2 Field assessment of soils
The results of the soil assessment are illustrated in the soil map in Figure 1. Photographs of fieldconditions are shown in Figures 2 and 3.
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Figure 1. Soil map of the investigated areas. The soil form and family are given for each test pit, with the effective soil depth in brackets after.
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Soil SuitabilityLow (3.0)Low (3.5)Low (4.0)Medium (4.5)
Figure 2. Photograph of field conditions on site.
Figure 3. Photographs of representative soil profiles. Left is a deep, sandy soil and right a shallowerone, with underlying clay.
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The geology of the area is transported sands overlying rocks of the Malmesbury Group. The soilsare generally duplex soils with light grey coloured, course, sandy upper soil horizons(approximately 3% clay) overlying a dense, clay horizon in the subsoil (approximately 40% clay).The depth to the clay horizon varies from 30 to >150 cm below the surface. The soils have drainagelimitations in that the subsoil is less permeable and water in the soil profile tends to build upabove the clay layer and drain laterally down-slope within the soil profile.
The general agricultural suitability of the soils is rated as low between 2.8 and 4.5 out of 10,according to the soil suitability rating system used by soil scientists in the Western Cape. Details ofthis system are provided in Table 3. Soil potential or suitability is a function of four factors: rootdevelopment potential (rooting depth and friability); water holding capacity and supply; drainage;and organic matter content. Soils are rated for potential by making an overall assessment of eachsoil, taking all of these factors into account.
Table 3. Interpretation of soil suitability ratings used by Western Cape soil scientists
Rating General agricultural suitability and recommendations forcultivated crop production
≤2 Very low Not recommended
>2 - ≤4 Low Not recommended
>4 - ≤5 Medium Conditionally recommended
>5 - ≤6 Medium-high Recommended
>6 - ≤8 High Highly recommended
>8 Very high Highly recommended
Soils are limited in terms of agricultural suitability by the leached, sandy nature of their upper soilhorizons, by their drainage limitations, and in some of the soils, by their limited depth due to theabrupt, shallow transition to the dense clay horizon in the subsoil. The dominant soil form isKroonstad, but Klapmuts, Fernwood, Katspruit, Westleigh and Dresden soil forms also occur.
Although the soils have low general agricultural suitability, they are nevertheless suitable forkikuyu pastures which are tolerant of the soil limitations.
No visual signs of impaired soil physical conditions as a result of irrigation, such as surface sealing,were evident in the investigated soils.
3.3 Chemical assessment of soils
The soil chemical analyses are presented in Appendix 1.
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In terms of impact on soil quality, the only parameter of possible concern is the salinity and relatedsodicity. The salinity, as indicated by soil resistance, is higher than ideal for sample 3. In the othersamples it is acceptable. Sample 3 is an area of shallow clay soil which will retain salinity muchmore than the other areas, that have deeper, sandy upper soil horizons, in which the salinity willtend to leach out. The levels of salinity in sample 3 are not problematic for kikuyu pastures butmay negatively impact salt sensitive crops.
The irrigation has had an impact on the sodicity of the soil. This is evident in that the exchangeablesodium percentage (ESP) is elevated in the soil samples, from what would be expected undernatural conditions, except in sample 2. The problem with high ESP is that it can cause impaired soilphysical conditions, most importantly, in the form of reduced infiltration rate due to surfacesealing, but also as reduced hydraulic conductivity and an increased tendency for hardsetting.These can have a negative impact, most importantly, on water infiltration. However, course, sandysoils, such as those on the site, are not sensitive to these impacts. Furthermore, the surface coverwith kikuyu grass also significantly reduces the risk of such impacts. As was stated in Section 3.2,no visual signs of impaired soil physical conditions, such as surface sealing, were evident in theinvestigated soils. Therefore, although the ESP of the soil has become elevated due to irrigationwith the effluent, it has not had a significant negative impact on the soil quality in terms of itsagricultural suitability for kikuyu pastures.
P and K are also elevated above natural conditions due to the irrigation, but this only serves toincrease the nutrient status of the soils and is likely to only benefit the growth of kikuyu pastures.This may be a limitation to crops that have a higher sensitivity to nutrient balances, but that is notrelevant to the soil suitability on the site.
pH is low in the soil samples, which is evidence that the higher pH of the effluent irrigation has hadno impact on the soil pH.
4 Conclusions
The soils are sandy with drainage limitations and have a low agricultural suitability. They are notsuitable for crop production but are suitable for kikuyu pastures.
An assessment of the water quality of the effluent irrigation showed that the only parameter ofpossible concern is the salinity and related sodicity of the water. All of the other water qualityparameters will have no impact on soil quality. The salinity, although higher than ideal, will have nolong term impact on soil quality and is suitable for the irrigation of kikuyu pastures. Although thesodicity has increased the exchangeable sodium percentage of the soil, this is highly unlikely tohave any long term impact on soil quality, because of the course, sandy texture of the soils and thesurface cover of kikuyu pastures.
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The irrigation with effluent has elevated the P and K levels in the soil, but this only serves toincrease the nutrient status of the soils and benefit pasture growth.
The conclusion of this assessment is therefore that irrigation with effluent water has not negativelyimpacted the soil quality, and is not likely to in the long term, and can therefore continue withoutthe need for any mitigation. However, some monitoring should be done as a precautionarymeasure. Comparative soil samples should be taken every three years to a depth of 30cm with theaim of identifying any trends in the exchangeable sodium percentage in the soil.
5 References
Department of Water Affairs and Forestry, 1996. South African Water Quality Guidelines (secondedition). Volume 4: Agricultural Use: Irrigation.
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Appendix 1 – Laboratory Soil Analysis
