Winery wastewater irrigation (Part 3): Soil chemical responses on a well-drained soil

Abstract

Most wineries in South Africa dispose their winery wastewater (WWW) through land application, but this results in the accumulation of soil potassium (K⁺) and sodium (Na⁺). This accumulation can affect soil structural stability and hydraulic conductivity. Taking above-mentioned into consideration, the objective of the study was to investigate the effect of WWW irrigation on the soil chemical properties at an existing grazing paddock at a winery near Rawsonville where WWW has been applied for many years. Results showed that due to the high volumes of WWW irrigation plus rainfall, large amounts of cations, particularly K⁺ and Na⁺, were leached beyond the 90 cm depth in the Longlands soils. Unfortunately, the leached elements will likely end up in natural water resources in the long run. Land application of WWW did not have a pronounced effect on soil pH_(KCl). The study confirmed that injudicious irrigation with untreated WWW poses a serious environmental hazard, particularly where crops in sandy soils are irrigated.

Consequently, land disposal of WWW by means of irrigation is definitely not the ultimate solution to the problem and can only be recommended where the WWW application does not exceed the water requirement of the grazing crop. Wastewater application according to the K⁺ requirement of the crop is also very crucial. This means that the WWW needs to be distributed on an area of land that is big enough so that the daily applications do not cause over-irrigation. Therefore, sound wastewater management can only be achieved by means of irrigation scheduling based on frequent soil water content measurements. Care should be taken that the irrigation water does not leach beyond the root zone. The soil chemical status should be determined at least annually. Depending on the type of soil and quality of wastewater, each winery will need to determine the size of land needed for irrigation with WWW high in K⁺. The winery will also have to consider the electricity costs if wastewater needs to be pumped from nearby farms in order to be utilised for a crop requirement. The effects of K:Na ratio in diluted or undiluted WWW on soil structure stability, K⁺ availability and leaching of elements also need to be addressed by continued research. Since the climate, particularly rainfall, will affect the accumulation and/or leaching of the elements, it is important that the research is carried out in field studies.

Introduction

More than 95% of South African wineries currently irrigate their wastewater onto land through sprinkler systems.¹ In this regard, in South Africa, the regulations for irrigating with winery wastewater (WWW) are contained in the General Authorisations for irrigation with wastewater.²

It is well known that WWW contains high levels of potassium (K⁺) and sodium (Na⁺), because wineries use K⁺ and Na⁺-based cleaning agents.¹ Therefore, using WWW for land disposal adds substantial amounts of these salts to the soil. Irrigation with WWW could be beneficial to overall soil fertility, although long-term application could have negative effects on soil chemical properties.^3,4,5,6 Land application of wastewater can increase the levels of soluble and exchangeable forms of K⁺ more rapidly than with conventional inorganic fertilisers.⁷

A survey carried out in South Africa to assess the soil chemical status where WWW had been disposed over prolonged periods showed that such irrigation increased soil K⁺ to a depth of 90 cm.⁸ In addition, the land disposal of WWW almost inevitably induced negative effects, irrespective of soil type. Furthermore, it was concluded that (i) in general, effluent disposal is poorly planned and managed, and disposal sites rarely seem to have been properly selected, because their soil properties are inappropriate for effluent disposal. In particular, deep sandy soils are unsuitable for disposal by ponding, mainly because of their high infiltration rates (IR), high permeability and low water storage capacity, and (ii) many disposal sites are too limited in area to permit the large volumes of effluent to be absorbed without surface runoff. This problem persists despite the presence of Kikuyu swards and sandy subsoil.

In pastures irrigated with WWW for over 100 years, total organic carbon (TOC), nitrogen (N), K⁺, Na⁺, calcium (Ca²⁺) and magnesium (Mg²⁺) levels increased relative to the control.⁹ Although soil K⁺, Na⁺, Ca²⁺ and Mg²⁺ of pastures irrigated with WWW for 15 to 20 years increased, these increases were not as substantial as where pastures had been irrigated for 100 years. Irrigation using WWW increased soil TOC.⁴ In addition, soil K, as well as salinity and sodicity levels, were higher in wastewater-treated plots compared to control plots, particularly woodlot and pasture sites at certain wineries.

Taking above-mentioned into consideration, the objective of the study was to investigate the effect of WWW irrigation on the chemical soil properties and potential environmental impacts at an existing grazing paddock at a winery near Rawsonville where WWW has been applied for many years.

Materials and methods

Details of the experimental site in an existing cultivated pasture grazing paddock where WWW had been applied for over 15 years have been given previously.¹⁰ The trial layout, description of the application of the WWW to the experimental site, as well as water quality, have also been given previously.

Characteristics and properties of the soil at the Rawsonville site

The soils around Rawsonville were formed from the alluvium of the Breede River and are relatively young. The soil at the site selected for the study showed no clear stratification (Figure 1) and contained a mottled subsoil, thus qualifying it for inclusion in the Longlands soil form¹¹ or a Gleyic, Albic, Arenosol.¹² The apedal soil consisted of fine sand. The B-horizon showed few fine mottles with distinct contrast and brown colour.

FIGURE 1. The Longlands soil form near Rawsonville showing no clear stratification.

Soil sampling and analysis

Soils were collected as described previously.¹⁰ Briefly, initial samples were collected before the start of the study in March 2011. Thereafter, samples were collected twice a year at 0-10 cm, 10-20 cm, 20-30 cm, 30-60 cm and 60-90 cm depth layers. All analyses were carried out by a commercial laboratory according to methods described previously.¹⁰

Results

Initial soil chemical status

After continuous irrigation with WWW for 15 years, the soil was acidic throughout the profile, i.e. the pH_(KCl) was less than 4.5 (Table 1). The soil Bray II P was high in all soil layers, i.e. more than 20 mg/kg which is considered to be the norm for sandy soils The basic cations declined with depth. By far the highest concentration of all cations occurred in the 0-10 cm layer. These levels were relatively high for sandy soils.¹³ This suggested that the sludge probably had a high CEC. The Ca_extr was the dominant cation, whereas Na_extr was the lowest throughout the profile (Table 1). The extractable potassium percentage (EPP’) was relatively high in the deepest soil layers. In contrast, the extractable sodium percentage (ESP’) was highest near the soil surface.

Soil potassium, sodium and pH

High amounts of WWW irrigation were applied in the course of the study (Figure 2A). The application of WWW increased the K⁺_extr levels in the 0-10 cm layer, and, to some extent in the 10-20 cm layer, at the end of the harvest periods (Figure 2B). Despite the seasonal fluctuations, K⁺_extr steadily increased over the three years in the first two soil layers compared to the levels at the beginning of the study. After three years of WWW application, there was no significant increase in K⁺_extr levels deeper than 20 cm depth (Figure 2B).

Similar to K⁺_extr, irrigation with WWW increased the Na⁺_extr levels in the 0-10 cm and in the 10-20 cm layers, at the end of the harvest periods (Figure 2C). In May 2012, the Na⁺_extr was also slightly higher in the 20-30 cm layer compared to the rest of the study period. Despite the seasonal fluctuations, Na⁺_extr tended to increase slightly over the two-and-a-half-year study period in the first two soil layers compared to the levels at the beginning of the study. At the end of the study period, there was no increase in Na⁺_extr deeper than 20 cm depth (Figure 2C). Since there was little change in Na⁺_extr levels with depth throughout the profile, it suggested that most of the applied Na⁺ was leached beyond 90 cm.

Irrigation with WWW increased the soil pH_(KCl) slightly until May 2012 (Figure 2D). In November 2012, the soil pH_(KCl) showed a decrease and tended to remain constant until November 2013. Variation in soil pH_(KCl) was not related to variation in monovalent cations. However, addition of organic acids from WWW could be associated with the decrease of soil pH due to H⁺ dissociation from carboxyl functional groups.¹⁴ While the soil pH increase could be associated with high concentration of total alkalinity in wastewater that contains bicarbonate ions, as well as deprotonated organic acids, the charge of these ions are countered by cations. When applied to soils it increases the pH due to anion hydrolysis reactions and decarboxylation.¹⁵ It is important to note that the soil was too acidic for viticulture, i.e. pH less than 5.5.¹³

FIGURE 2. Temporal variation in (A) rainfall and winery wastewater irrigation, (B) soil K⁺, (C) soil Na⁺, and (D) soil pH_(KCl) where winery wastewater was applied to a Longlands soil near Rawsonville.

Soil EPP’ and ESP’

With the exception of the 0-10 cm layer, the EPPʹ tended to be lower at the end of the harvest period, followed by an increase during winter (Figure 3A). This result is somewhat unexpected since the higher EPPʹ did not correspond with the higher K⁺ applications which caused higher K⁺_extr in the soil (Figure 2B). Although substantially more K⁺ than Ca²⁺ was applied via the WWW, Ca²⁺ was the dominant cation in all the soil layers except in November 2013 when the Ca²⁺_extr levels were comparable to the other extractable cations in the deeper layers (Figure 4A). The source of the Ca²⁺ was probably lime that was added to the WWW in order to increase the pH as part of the wastewater treatment carried out by the winery. Routine use of Ca²⁺ amendments including, yet not restricted to lime, gypsum and calcium nitrate, either added directly to wastewater or to soils will enable Ca²⁺ exchange and displacement of Na⁺ and K⁺. Winter application of Ca²⁺ amendments will ensure its percolation down the soil profile thereby ensuring good distribution of Ca²⁺.¹⁶ Quantification of this practice was beyond the scope of the study. In November 2013, the winery probably reduced or stopped the lime application, which caused the low soil Ca²⁺_extr. Based on the foregoing, it seemed that high levels of Ca²⁺_extr at the end of the harvest dominated the exchange complex to such an extent that the EPPʹ was reduced compared to the winter when the Ca²⁺_extr was lower. The high EPPʹ in November 2013 was due to the low Ca²⁺_extr. These results also suggested that the large amounts of applied K⁺ via the WWW were not preferentially absorbed onto the exchange sites.

Although the Na⁺_extr showed some seasonal fluctuations, it did not reflect in the ESPʹ (Figure 3B). The lack of seasonal fluctuations in ESPʹ was probably due to the dominance of Ca²⁺_extr, and to some extent K⁺_extr. It was previously reported that the adsorption of Na⁺ on soils similar to the Longlands soil was reduced by the presence of high levels of K⁺ after winery wastewater irrigation.¹⁷ High soil ESPʹ increases the risk of soil physical properties to deteriorate through clay dispersion which will lead to structural breakdown and blockage of soil pores and reduced soil permeability.¹⁸ However, since the ESPʹ was relatively low, it would probably not have caused serious soil physical deterioration.

FIGURE 3. Temporal variation in soil (A) EPP’ and (B) ESP’ where winery wastewater was applied to a Longlands soil near Rawsonville.

Soil calcium and magnesium

The Ca_extr in the 0-10 cm and 10-20 cm layers, and to a lesser extent in the 20-30 cm layer, tended to increase at the end of the harvest period (Figure 4A). This was followed by a decline during winter. It is interesting to note that the seasonal variation in Ca²⁺_extr occurred in the 30-60 cm layer although the concentrations were considerably lower compared to the topsoil. A previous study showed that continuous application of WWW high in K⁺ and Na⁺ could cause the soil exchange sites to be dominated by monovalent ions, thereby pushing bivalent ions such as Ca²⁺ and Mg²⁺ out of the exchange complex.⁶ Consequently, the bivalent cations could be leached from the soil. However, the Ca_extr in the deeper layers remained constant throughout the study period under the prevailing conditions. Although Ca²⁺ levels were generally low in the WWW, it seemed that higher applications during the harvest period were reflected in the Ca_extr. Since the applied Ca²⁺ was substantially lower than amounts of K⁺ and Na⁺, it is unlikely that the Ca²⁺ would affect the EPPʹ or ESPʹ significantly. Therefore, the bivalent cations will probably not counter structural problems caused by high amounts of K⁺ and Na⁺ from the WWW when applied to the soil.

The Mg²⁺_extr in the 0-10 cm, and to a lesser extent in the 10-20 cm layer, showed the same seasonal fluctuation as the Ca²⁺_extr (Figure 4B). The Mg_extr in the deeper layers remained more or less constant throughout the study period. Although Mg⁺ levels were generally low in the WWW, it seemed that higher applications during the harvest period were also reflected in the Mg_extr. Similar to Ca²⁺, the low levels of Mg²⁺ are unlikely to counter the negative effects of high K⁺ and Na⁺ applications on EPPʹ or ESPʹ, and consequently on soil physical conditions.

FIGURE 4. Temporal variation in soil (A) Ca²⁺ and (B) Mg²⁺ where winery wastewater was applied to a Longlands soil near Rawsonville.

Soil phosphorus

The soil P fluctuations appeared to be erratic (Figures 5 and 6). At certain times, the P in the topsoil tended to increase, whereas the subsoil P tended to decline and vice versa. Therefore, it seemed that leaching of P into the subsoil occurred, which coincided with P losses from the topsoil. This was illustrated more clearly when the means for the topsoil (0-30 cm depth) and subsoil (30-90 cm depth) were plotted over time (Figure 6). It seemed that the increase in subsoil P lagged behind P increases in the topsoil up till November 2012. Following this, top and subsoil fluctuations coincided until November 2013. The high rainfall and irrigation before May 2013 probably caused leaching of P throughout the soil profile. However, this does not rule out the possibility that the low pH reduced the solubility of the P.

FIGURE 5. Temporal variation in soil P where wastewater was applied to a Longlands soil near Rawsonville. Dashed line indicates the proposed P norm for grapevines (Conradie, 1994).

The soil P content was substantially higher than the minimum requirement for vineyards.¹³ It must be noted that leaching of high levels of P into groundwater, as well as other freshwater sources close to the winery, could cause serious environmental problems, e.g. eutrophication. The leaching of P poses a very serious risk to the nearby water streams. Due to the sandy nature of the soil, i.e. 3.3% clay, and low Fe content, it does not have adequate P adsorbing capacity.¹⁹ This would increase the risk of leaching excessive P from the soil.

FIGURE 6. Temporal P-variation in the topsoil (0-30 cm) and subsoil (30-90 cm), as well as irrigation plus rainfall, where wastewater was applied to a Longlands soil near Rawsonville. Vertical columns indicate irrigation plus rainfall.

Soil nutrient balances

Since there was little change in K⁺ levels with depth throughout the profile, it suggested that most of the applied K⁺ was leached beyond 90 cm. In fact, seasonal soil K⁺ balances showed that substantial amounts of K⁺ were leached (Table 2). Furthermore, the cumulative leached K⁺ was linearly related to the cumulative irrigation plus rainfall (Figure 7). Due to the low clay content of the soil, the exchange complex could not retain large amounts of K⁺. Therefore, leaching of K⁺ beyond 90 cm was not inhibited. Although leaching of K⁺ from sandy or coarse-textured soils during winter rainfall reduces the risk of accumulation and clay dispersion, it increases environmental risks such as groundwater recharge and/or lateral flow into other freshwater resources.

FIGURE 7. Effect of cumulative (Σ) irrigation plus rainfall on cumulative K⁺ losses beyond 90 cm depth where a Longlands soil was irrigated with winery wastewater near Rawsonville.

Seasonal soil Na⁺ balances confirmed that substantial amounts of Na⁺ were leached (Table 3). Furthermore, the cumulative leached Na⁺ was also linearly related to the cumulative irrigation plus rainfall (Figure 8). Similar to K⁺, the low clay content of the soil could probably not retain large amounts of Na⁺. Therefore, leaching of Na⁺ beyond 90 cm was also not inhibited. Although leaching of Na⁺ from sandy or coarse-textured soils during winter rainfall also reduces the risk of accumulation and dispersion, it poses the same environmental risks as the large amounts of K⁺ that were leached from the soil. High concentrations of Na⁺ in soil due to WWW application can reduce soil aggregate stability.¹⁶ When Na⁺ is the predominant adsorbed cation, the clay disperses. When the soil is wet, puddling reduces permeability, and when it is dry, a hard impermeable crust forms.

Since there was little change in Na⁺_extr levels with depth throughout the profile, it suggested that most of the applied Na⁺ was leached beyond 90 cm. Seasonal soil Na⁺ balances confirmed that substantial amounts of Na⁺ were leached (Table 3). Furthermore, the cumulative leached Na⁺ was also linearly related to the cumulative irrigation plus rainfall (Figure 8). Similar to K⁺, the low clay content of the soil could probably not retain large amounts of Na⁺. Therefore, leaching of Na⁺ beyond 90 cm was also not inhibited. Although leaching of Na⁺ from sandy or coarse-textured soils during winter rainfall also reduces the risk of accumulation and dispersion, it poses the same environmental risks as the large amounts of K⁺ that were leached from the soil. High concentrations of Na⁺ in soil due to WWW application can reduce soil aggregate stability.¹⁶ When Na⁺ is the predominant adsorbed cation, the clay disperses. When the soil is wet, puddling reduces permeability, and when it is dry, a hard impermeable crust forms.

FIGURE 8. Effect of cumulative (Σ) irrigation plus rain on cumulative Na⁺ losses beyond 90 cm depth where a Longlands soil was irrigated with winery wastewater for two and a half years near Rawsonville.

Conclusions

It is important to note that the study represented the worst-case scenario, i.e. the WWW disposal was carried out in a smaller paddock. Due to the high volumes of WWW irrigation plus rainfall, the inevitable over-irrigation leached large amounts of cations, particularly K⁺ and Na⁺, beyond 90 cm depth in the Longlands soils. Unfortunately, the leached elements are bound to end up in natural water resources in the long run. Irrigation with the WWW did not have a pronounced effect on soil pH_(KCl). The study confirmed that injudicious irrigation with untreated WWW poses a serious environmental hazard, particularly where crops in sandy soils are irrigated.

Due to the risks involved as discussed above, disposal of WWW by means of irrigation is definitely not the ultimate solution to the problem. Land disposal can only be recommended where the WWW application does not exceed the water requirement of the grazing crop, or any other agricultural crop. Wastewater application according to the K⁺ requirement of the crop is also very crucial. This means that the WWW needs to be distributed on an area of land that is big enough so that the daily applications do not cause over-irrigation. Therefore, sound wastewater management can only be achieved by means of irrigation scheduling based on frequent soil water content measurements. Care should be taken that the irrigation water does not leach beyond the root zone. The soil chemical status should be determined at least annually. The basis to which wastewater should be applied for a given crop should be based on water and nutrients requirement such as K⁺. Depending on the type of soil and quality of wastewater, each winery will need to determine the size of land needed for irrigation with WWW high in K⁺. The winery will also have to consider the electricity costs if wastewater needs to be pumped from nearby farms in order to be utilised for a crop requirement.

Based on the foregoing, it is essential that future research should focus on selecting halophytic crops that are capable of absorbing the applied elements, particularly K⁺ and Na⁺, if land disposal of WWW is the only option. Preferably, the foliage and roots or tubers should be removed from the land when the crop is harvested. The effects of K:Na ratio in diluted or undiluted WWW on soil structure stability, K⁺ availability and leaching of elements also need to be addressed by continued research. Since the climate, particularly rainfall, will affect the accumulation and/or leaching of the elements, it is important that the research is carried out in field studies.

Acknowledgements

• This article is an output of WRC Project K5/1881, entitled “The impact of wastewater irrigation by wineries on soils, crop growth and product quality”. This solicited project was initiated, funded and managed by the WRC. The project was co-funded by Winetech and ARC.
• Goudini and Koelenhof wineries for their permission to work at their land and utilisation of their wastewater for research.
• ARC for infrastructure and resources.
• Staff of the Soil and Water Science division at ARC Infruitec-Nietvoorbij for their assistance, and in particular Mr. F. Baron for his dedicated technical support.

References

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For more information, contact Reckson Mulidzi at mulidzir@arc.agric.za.

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