ASSESSMENT OF ENVIRONMENTAL IMPACTS OF GROUNDWATER ABSTRACTION FROM TABLE MOUNTAIN GROUP (TMG) AQUIFERS ON ECOSYSTEMS IN THE KAMMANASSIE NATURE RESERVE AND ENVIRONS
Background and Motivation
The effect of large-scale groundwater abstraction on the environment is largely unknown as very few studies in this field have been conducted in South Africa. The Klein Karoo Rural Water Supply Scheme (KKRWSS) that abstracts water from the Table Mountain Group aquifers, is situated in the Little Karoo, South Africa. This abstraction has been effected for seven years. A previous study funded by the Water Research Commission (WRC) on the geohydrology of the western section of the Kammanassie Mountain recommended that the impact of water abstraction on the environment should be investigated. As a result WRC initiated this study to investigate the effects of large-scale groundwater abstraction of this scheme on the environment.
Objectives of the project
Climate change in the Little Karoo
The Little Karoo is regarded as a dry region averaging monthly totals of less than 31 mm per month. Rainfall over the region does not exhibit a distinctive bimodal seasonal cycle.
A long rainfall record (1925 to 2000) reveals that the Little Karoo experienced unusually high rainfall during 1981, 1985 and 1996. These high rainfall values contribute to a general positive rainfall trend since 1925. If the shorter period of the recent 30/31 years (1971 to 2000/1) is considered, a general negative trend appears, implying that in both the Oudtshoorn and Kammanassie areas a decrease in the amplitude of the rainfall variability occurred over the past thirteen years. The trend is most obvious in the DJF, MAM and JJA seasons while rainfall totals increased over the SON season. The negative trends are not unusual and can be regarded as a return of rainfall totals to the longer-term norm (see Hoekplaas analysis - figures 3.4 and 3.5). A strong deviation from the norm was rather the extreme wet years that occurred in the 1970s and 1980s.
The more recent period of smaller rainfall deviations (1989 to1999) might have contributed to less soil moisture than for the preceding extremely wet period. In particular, a sequence of below-normal rainfall years appears in the JJA record (1985 to 2000). The period of smaller deviations does not necessarily point towards a drought, but compare well with rainfall variability experienced before the 1970s.
Impacts of groundwater abstraction on springs on the Kammanassie Mountain Groundwater abstraction by the KKRWSS has impacted on the low-flow discharging of the Vermaaks River. Base flow in the Huis River has most probably been influenced by abstraction from the well field though the volume cannot be quantified. Superimposed on this abstraction is a declining precipitation trend since the commissioning of the well field. Under natural conditions permanent water occurs at two localities in the Vermaaks River Valley where springs emanated in the river course. These springs support localised ecosystems. Abstraction dried up one of these "permanent water" localities (spring 009) causing localised impact. The other spring (051) was temporarily affected and stopped flowing for 6 months. In the majority of cases the combined effect of the negative rainfall trend over the past 13 years is the probable cause of springs drying up. There appears to be a lag period (7 years) between the start of abstraction and a resultant significant impact on surface flow in the Vermaaks River.
Of the 53 springs on the Kammanassie Mountain Range, nine fall into the most vulnerable category, 10 into the intermediate vulnerability and the remaining 34 are the least vulnerable to the influence of abstraction. Twenty-seven springs (50%) on the Kammanassie Mountain clearly emanate from perched groundwater systems (type1), which cannot be influenced by groundwater abstraction and are excluded from potential influence. Sixteen (30%) "water table" (type 2) springs occur and are potentially vulnerable to the effects of abstraction if all the other hydrogeological parameters permit. In a further 10 cases (19%) there is a possibility that the springs emanate from perched systems but there is an element of doubt. Thirteen (48%) of the perched groundwater table springs have dried up since the well field was established indicating their susceptibility to low/irregular recharge from rainfall and snow. Of the 9 most vulnerable springs, 3 are excluded from influence, as they are perched. Only one has definitely dried up as a result of abstraction. A further two of these springs are still flowing. In the remaining 4 cases there is a strong case to suggest other factors (agricultural, other scheme well fields and climate) have played a significant role in springs drying up.
Of the 10 intermediate vulnerability springs, 3 have been disqualified on the basis of timing of dry up and one on the basis of being perched. In the remaining 6 cases there are strong indications that influences other than Vermaaks well field abstraction have played a major role in springs drying up. It has long been recognised that the Cedarberg shale plays a major role in the occurrence of springs. It has become clear that a large portion of these springs emanates from perched groundwater tables and is therefore not vulnerable to influences of abstraction.
This research has only focused on the impact of abstraction of 0.6 million m3/a from the Vermaaks river well field. It has to be recognised though that other groundwater users tapping this resource will also impact the aquifer and potentially influence surface/near surface flow.
Impacts of groundwater abstraction on the vegetation of the Vermaaks and Marnewicks Valleys
In order to determine suitable control sites for plant water stress tests, TWINSPAN classification, refined by Braun-Blanquet procedures were carried out in the Vermaaks, Marnewicks and Buffelsklip Valleys. Twenty-seven plant communities, which could be grouped into 13 major communities were identified. A classification and description of these communities as well as a vegetation map are presented. The diagnostic species as well as the prominent and less conspicuous species of the tree, shrub, herb, restio and grass strata are outlined.
A checklist was produced for the Vermaaks, Marnewicks and Buffelsklip Valleys to determine the plant species richness for these areas in order to compare these sites for plant water stress tests. A total of 481 plant species were found for the Vermaaks, Marnewicks and Buffelsklip Valleys while a new Erica species was found at Buffelsklip.
The total of 441 plant species was identified in the Vermaaks Valley, which represent 229 genera and 76 families. Flowering plants are represented by Monocotyledoneae with 98 species in 16 families, Eudicotyledoneae with 329 species in 53 families and the Palaeodicotyledons with 1 species and 1 family. The Pteridophytes with 13 species in 9 families represent the non-flowering plants. A total of 189 plant species were recorded in the Marnewicks Valley, which represent 120 genera and 57 families. Flowering plants are represented by Monocotyledoneae with 42 species in 10 families and Eudicotyledoneae with 145 species in 44 families The Pteridophytes with 3 species in 3 families represent the non-flowering plants. A total of 171 plant species were recorded in the Buffelsklip Valley representing 118 genera and 55 families. Flowering plants are represented by Monocotyledoneae with 38 species belonging to 12 families and Eudicotyledoneae with 128 species and 40 families. The Pteridophytes with 5 species and 3 families represent the non-flowering plants.
Thirteen families dominate the Vermaaks, Marnewicks and Buffelsklip flora. The two largest families are the Asteraceae with 77 species and Poaceae with 40 species. A total number of six red data species were recorded.
Three experimental sites were selected in the Vermaaks Valley with control sites at the Marnewicks and Buffelsklip Valleys. Vermaaks 1 was at the site of abstraction; Vermaaks 2 was downstream of Vermaaks 1, where a spring had dried up in 1999 and Vermaaks 3 was downstream of Vermaaks 2, where a spring was still flowing. The Marnewicks Valley is similar to the experimental sites in geology, climate, topography and vegetation. The Buffelsklip site was found to be an unsuitable control site as rainfall and water table readings were found to vary considerably from the Vermaaks and Marnewicks Valleys.
At each of the 5 sites, five individuals of Rhus pallens, Dodonaea angustifolia, Acacia karroo, Nymania capensis and Osyris compressa were randomly selected and permanently marked for the duration of the study. Three types of plant water stress tests were carried out on these individuals namely: plant moisture stress (Shölander pressure gauge); leaf water content (leaf disc method) and stomata resistance (porometer). Tests were carried out in spring, summer and winter over a 2-year period.
ANOVA, t-test (parametric), Mann-Whitney and Kruskal Wallis tests (non-parametric) were used to ascertain whether there were significant differences (p-value<0.05), per season, per species, per test between the following sites: Vermaaks 1 and Marnewicks; Vermaaks 2 and Marnewicks; Vermaaks 3 and Marnewicks (t-tests and Mann-Whitney) and all 4 sites (ANOVA and Kruskal Wallis). Boxplots (Box-and-whisker diagram) were used to graphically depict the results of the test results obtained. Pearson's Product Moment correlation coefficients were calculated to determine whether there were any correlations between the plant water stress tests (plant moisture stress, stomata resistance and leaf water content) per season, per species.
A 24-hour study was carried out to determine the most suitable time to collect plant material for plant water stress tests. Results indicated that variables such as (light, temperature, wind and relative humidity) were most stable between 22:00 and 03:00. Therefore plant material for plant moisture stress and leaf water content tests were collected simultaneously at each site at 22:00. The time most suitable for Porometer readings was found to be between 10:00 and 14:00.
All species except Nymania capensis, showed similar trends in plant moisture stress and leaf water content stress tests for spring, summer and winter, with more pronounced results in summer. The karroid plant Nymania capensis was found to be an unsuitable species for plant water stress tests. This could be ascribed to this plant probably having anatomical or physiological adaptations to drought, particularly in summer. Furthermore porcupine damage to the base of the stem of this species could also have induced plant moisture stress. The plant species Rhus pallens, Dodonaea angustifolia, Acacia karroo and Osyris compressa were found to be suitable species for plant moisture stress tests. Higher plant water stress in summer could be attributed to drier conditions (less rainfall) and increased groundwater abstraction.
The control site at Marnewicks was less stressed than Vermaaks 1, 2 and 3. Vermaaks 3 was less stressed than Vermaaks 2, while Vermaaks 1 was the most stressed. Higher stress at Vermaaks 1 is probably caused by drier conditions of the ecosystem at this site. The drier ecosystem, as a result of higher altitude, comprises more open woody vegetation. Groundwater abstraction also caused a drop in the water table (34.64 - 60 metres below surface).
According to the statistical analysis there is significant plant water stress in Vermaaks 2 compared to the control site at Marnewicks. Vermaaks 3 also shows higher plant water stress than Marnewicks (the control), especially in the summer months, though less than Vermaaks 2. Vermaaks 3 is further away from abstraction than Vermaaks 2 and also falls within the alluvium basin. Plant water stress at Vermaaks 2 and 3 is partially due to groundwater abstraction as these sites receive similar rainfall to Marnewicks, which was the least stressed site. The significant plant water stress results for summer, that revealed Vermaaks 2 & 3 as significantly more stressed than Marnewicks, seem to indicate that groundwater abstraction (elevated during the summer months) places additional water stress on vegetation during their growing season. The data from Vermaaks 2 & 3 seem to indicate that groundwater abstraction, superimposed by decreased rainfall and recharge, has a significant negative impact on plant water stress at the experimental sites in the Vermaaks River Valley. Changes in the water abstraction management could perhaps improve the situation.
There was a negative correlation between plant moisture stress and leaf water content tests for Rhus pallens, Dodonaea angustifolia, and Nymania capensis. Acacia karroo and Osyris compressa showed a negative correlation between stomata resistance and leaf water content.
Data obtained from the porometer readings varied. This can be ascribed to the dependency of stomata closure on light intensity, wind and temperature. Statistical analysis (Pearson Correlations) between stomata resistance and light intensity confirmed this correlation. Because of the limitations of available equipment, porometer readings could not be measured simultaneously at all sites. This resulted in a great variation in the porometer readings. It was therefore decided to discard these results.
Impacts of groundwater abstraction on the vegetation of springs on the Kammanassie Mountain
A checklist was produced to determine the plant species richness for springs on the Kammanassie Mountain. A total of 244 plant species were recorded for the springs, which represent 145 genera and 71 families. Flowering plants are represented by Monocotyledoneae with 63 species belonging to 7 families and Eudicotyledoneae with 156 species and 43 families. The Pteridophytes with 12 species in 8 families represent the non-flowering plants. A total of 12 species in 12 families of Bryophytes were found. One species of Gymnosperm was found. The two largest families found were Asteraceae with 41 species and Poaceae with 25 species. Two red data species were found.Flowing springs have very different plant communities to those that are dry. A difference between how long a spring has been dry also determines the plant community present. Very strong differences occur in plant species in flowing springs with ferns, mosses and water plants dominating these springs. Springs, that have been dry for a number of years, have a shrub, grass and restio dominance. Springs that have dried up recently (within the past 3 years) show a distinct difference once again, with the herbaceous layer being dominant.
Impacts of groundwater abstraction on Cape Mountain Zebra on the Kammanassie Mountain
Most of the Cape mountain zebra today, live in seeded populations, most of which were derived from the largest and most successful of the relict populations - the Cradock population of Mountain Zebra National Park. The two smallest relict populations in the Kammanassie and Gamka Mountains, both reduced to critically small numbers, may not have recovered from the extreme genetic bottlenecking. In the last 30 years, both these populations have not increased in number from 10 - 40 animals each. The Cape mountain zebra thus remains at the status of "endangered" in the IUCN's red data book of threatened species. Each stock population therefore represents 1/3rd of the entire Cape mountain zebra gene pool. This is alarming given that 2/3rds of the gene pool exists in only 5% of the metapopulation, i.e. 38 animals at Kammanassie Nature Reserve and 24 at Gamka Mountain Nature Reserve. In terms of conservation management, these two reserves are critically important in the maintenance of genetic diversity of Cape mountain zebras. A loss of one of these populations will reduce the genetic variation by a third.
There are currently 38 Cape Mountain Zebra on the Kammanassie Mountain that depend on springs for drinking water. A total of four Cape Mountain zebra died between November 2000 and August 2001. All four of these deaths were either directly or indirectly linked to springs drying up or to inaccessibility to flowing springs. Two artificial watering points have been installed to give foals and pregnant mares easy access to drinking water. Without artificial watering points at strategic places on the Kammanassie Mountain the 38 endangered Cape Mountain Zebra risk extinction as a result of natural water sources (springs) drying up.