*) Address of the author: Dr. STEPHAN KEMPE Geologisch-Paläontologisches Institut und Museum der Universität Hamburg, Bundesstraße 55, 2000 Hamburg 13, Bundesrepublik Deutschland
 

The dominating role of dissolved CO₂ in weathering and chemical erosion of continents is well established (e. g. GARRELS & MACKENZIE, 1971). However, textbooks on groundwater and karstology are principally concerned with the thermodynamics of CO₂ reactions but say little on specific geological situations, namely case studies (MATTHESS, 1973; RICHTER & LILLICH, 1975; PICKNETT et al., 1976; BÖGLI, 1978).

Prior to the use of portable pH-meters and fully computerized thermodynamic electrolyte models, CO₂ concentrations were often approximated by the TILLMANNS equation (TILLMANNS & HEUBLEIN, 1912):

[CO2] = K x [HCO3-]2 x [Ca ++]

(45)

which is a measure of the amount of CO₂ needed to hold a given amount of calcium carbonate in solution. The Tillmanns constant "K" is temperature dependent. To estimate the "aggressive" free CO₂, that is the CO₂ exceeding the amount calculated by the Tillmanns equation, a series of techniques were developed using pH-measurements or direct determination of the total free CO₂ (these techniques are discussed in detail by MATTHESS, 1973).

Monitoring of small PCO₂ and mineral saturation variations with time over a range of groundwater samples derived from various sources has become feasible relatively recently through the development of precise computerized thermodynamic calculations and sample evaluation. As the question of the degree of saturation with respect to carbonate minerals is especially interesting for the understanding of the geochemistry of karsts, accurate PCO₂ calculations are based largely on field studies from these areas (e. g. PARIZEK et al., 1971; HARMON et al., 1972; TRAILKILL, 1972; WIGLEY et al., 1973; HESS, 1974; FLEYFEL, 1979).

Saturation of carbonate minerals was related to the type of aquifer (PARIZEK et al., 1971), for instance diffuse flow through limestone results in higher carbonate mineral saturation and mostly higher PCO₂ when compared to conduit flow. In addition, mean annual temperature influences the mean PCO₂ (HARMON et al., 1972).

To illustrate the effect of temperature on PCO₂ we will briefly examine the region between Southern Mexico and the Northern United States where a groundwater temperature gradient of about 20° C exists. Due to higher respiration activity in the much warmer tropical and subtropical soils , the PCO₂ of groundwaters differs between the two areas by a factor of 10. In Mexico PCO₂ values of 30 000 ppm were measured compared to a mean of 3000 ppm in the Northern U.S. karst regions (HARMON et al., 1972). PARIZEK et al. (1971) and HESS (1974) also report substantial PCO₂ variations in karstic groundwaters, due to seasonal effects.

The first comparative study of PCO₂ from a variety of geologic settings in a Central European karst region was conducted by the author and co-workers in May 1973 in the Zechstein area South of Osterode/Harz, West Germany (BRANDT et al., 1976) (Fig. 2). A total of 41 water samples from gypsum-, limestone-, and nonkarstic waterbodies were analysed and the PCO₂ calculated. The results showed a large scatter of values between 300 and 20 000 ppm. Due to monoseasonal sampling it could not be decided if this scatter was due to genuine environmental differences in the sources sampled or due to different phase shifts in the seasonal PCO₂ curve for the individual sample locations.

To settle these questions and to obtain the absolute level of PCO₂ and the magnitude of seasonal fluctuations for individual water sources a one and a half year hydrochemical monitoring was conducted in the area between November 1974 and April 1976. The results of this field experiment are presented below.

Fig. 2 Location of the groundwater investigation area in the South Harz, Germany, and schematic geological profile through Zechstein formations.

The upper Permian (Zechstein) salinar series of the South Harz/Germany consists of four cyclic stratgraphic units (Table 4) (HERRMANN, 1957). South of Osterode (Fig. 2), the Zechstein forms a two to three kilometer wide NNW-SSE extending outcrop. The former halite has been removed by subrosional processes. Only the anhydrite (changed partly to gypsum), limestone, dolomite, and clay formations are exposed at the surface. Uplift of the Variscian folded Harz Mountains has slightly tilted the Zechstein toward the SE. Locally, however, the earlier subrosion of the halite and the ongoing dissolution of the gypsum and carbonate series has caused much irregularity in the outcrop pattern. In addition, NW-SE and ENE-WSW striking faults disturb the stratigraphic order. Fig. 2 gives a schematic profile of the Zechstein outcrop. The A1, the Werra Anhydrite forms a prominent white escarpment which is capped by the Ca-2, the Staßfurt-, Haupt-, or Stinkdolomite which in places is covered by the up to 15 m thick T 3, the Grauen Salzton. These two formations form the Plateau of Düna. Toward the south, the intercalated A 2 or Basalanhydrite is gradually vanishing. Faults have lowered the originally up to 70 m thick A 2 or Hauptanhydrite to the level of the Düna Plateau, and by that protected this formation from becoming eroded. The Zechstein suite is buried by the steep escarpment of the lower Buntsandstein which also hides the thin beds of the Zechstein-4 or Aller-series (KEMPE et al., 1970.

The Hauptanhydrite-Karst Hainholz and a smaller outcrop toward the NW - the Beierstein - form, together with the surrounding dolomite plateau and the clay and siltstone terrain of the Buntsandstein, a perfect hydrochemical test ground within a very small area. In addition, the karst has been declared a conservation area since 1967 minimizing the influence of human activity (KEMPE, 1979 c).

The general hydrochemistry has been worked out in 1973 (Fig. 3). A morphologic water devide between the Hackenbach to the North and the Sieber to the South, straddles the area, but underground direction of karst drainage remains largely unknown. For details see BRAND et al. (1976).

Apart from thePCO₂ the relation of the carbonate hardness versus the sulphate concentrations as shown in Fig. 4 is most informative for geologic interpretation.

Fig. 4 Average sulphate vs. average carbonate concentrations for 31 locations of the Hainholz/Beierstein area. Standard deviations are omitted for greater clarity in the gypsum karst water field.

Group A
Group A comprises the three PCO₂ top-ranking locations 10, 8, and 26 augmented by location 25 with a somewhat lower pressure.

Sample 10 is from a 3 m deep, 50 cm wide pot-hole in a recent tuffa cone of silt sized fine calcite. The „Arteser“ discharges water derived from the agricultually used Staßfurt-Dolomite plateau of Düna. The spring is located on the North-West Hainholz fault along which the Staßfurt-Dolomite waters spill over into the Hainholz gypsum. The carbonate concentration of the Arteser is 5 meq/l and the spring carries hardly any sulphate (compare Fig. 4). The Ca/Mg ratio is 6.5 which reveals that the facies of the Staßfurt-Dolomite in this area is calcareaus. The high PCO₂ of 20 000 ppm shows that the aquifer is well protected to soil air exchange. This is in accordance with the geologic situation: the Staßfurt-Dolomite plateau of Düna is at least partially covered by T 3 clay (see. Fig. 2).

The spring rapidly reacts to precipitation changes and the level of the 100 m distant sinkhole pond "Pferdeteich" (sample 11), follows the precipitation pattern with a time lack of about 10 days. Thus, the discharge from the limestone aquifer is affected at least partly by conduit flow. In spite of the high PCO₂ and CaCO₃ concentrations the water is generally not calcite saturated which also explains the presence of cave conduits allowing rapid water transfer in this limestone aquifer.

The locations 26 and 25 have the same geologic settings as location 10, in that they also shed water from the Staßfurt-Dolomite with high PCO₂ and carbonate concentrations. Location 25 is located in the bed of a creek water admixed to it. The overall PCO₂ of location 25 is therefore lower than normal for a A-Group. The sulphate content is higher in these springs, which increases the Ca/Mg ratio to over 10.

As annual mean both springs are supersaturated with calcite but not with dolomite. Individual samples, however, may show undersaturations. Due to varying proportions of creek and spring waters in location 25 the variation in PCO₂ all the year round is the largest of all locations.

Location 8 is a spring in the Holocene fill of the Beiersteinteich. It is almost saturated with gypsum and slightly undersaturated with respect to calcite. The high PCO₂ is most likely derived from the thick Holocene section (H.-J. WEINBERG, Hamburg, personal communication). The waters seem to be largely anoxic with substantiates this interpretation.

Group B
The second highest PCO₂ value group is formed by the three ponds studied here. Locations 2, 31 and 11 are sinkhole ponds changing their volume considerably during the year, often down to total dryness.

The Beiersteinteich (Location 2) with an average pressure of almost 13 000 ppm is rather shallow with a marsh bottom (see above) and acts as a spring for sulphate karst water percolating upward through its sediments for most of the year. During very dry conditions, however, it also serves as a sink for water from the Schurfbach to the southeast causing the creek to bifurcate (F. VLADI, Osterode, personal communication).

The high PCO₂ of the Beiersteinteich (Location 2) might be affected by CO₂ from the dissimilation of its organic fill. The other gypsum karst waters of the Beierstein (see Map Fig. 3 for locations) do not have such high values. The Beiersteinteich is clearly influenced by respiration CO₂ derived from its own waters and sediments.

Pond 22 in front of the Marthahöhle does not have any tributary. It is recharged from surface runoff and from gypsum karst water percolating from the side (Marthahöhle). The Marthahöhle has a much lower PCO₂ suggesting that this pond draws its CO₂ from internal respiration too.

The Pferdeteich as the sink for the Arteser - waters (Location 10) is the only one which undoubtedly receives water primarily high in PCO₂. Also its coefficient of variation is low. It therefore can be ruled out that its high PCO₂ is not only autorespirative in nature.

Group C
Group C includes 13 location with PCO₂'s between 5300 and 3000 ppm. All locations have a comparatively low coefficient of variation (Table 5). On the carbonate-sulphate diagram (Fig. 4) these locations cluster into an acra of median carbonate and middle to high sulphate concentrations derived directly from gypsum karst springs (Locations 21, 4, 19 forming the subgroup C 1) or are accessible inside the caves as pools (C 2 group).

Springs from gypsum karst have higher PCO₂ than cave waters. Cave pools have larger surfaces and CO₂ is constantly evading through them into the rapidly exchanged cave air. The cave pools further show considerable undersaturation with respect to gypsum. The waters encountered in the pools of the Jettenhöhle can only be derived from the underlying not-gypsiferous carbonate strata of the Platten-Dolomite (Ca 3) and Staßfurt-Dolomite (Ca 2).

No lateral water source is otherwise entering the Jettenhöhle and the seepage water is already saturated with gypsum upon reaching the karst water level (see Group F).

The Jettenhöhle seemingly owes its existence to a 'double karstification'. First the water derived from the Düna Plateau dissolved the limestone underneath the gypsum and becomes charged with carbonates. Along minor faults water upwells by buoyancy toward the water table which is within the gypsum. Water saturated with gypsum replaces the carbonate water in the lower aquifer, thus replenishing the cave pools with carbonate water. Due to the common ion effect of the calcium entering the solution by dissolution of the gypsum and due to a constant degassing of CO₂, the waters become supersaturated with calcite with precipitates from the cave pool. Several meters thick cave sediments of authigenic calcite are the result (KEMPE & EMEIS, 1981). Originally the cave totally developed below the watertable until breakdown began to enlarge the ceiling into higher levels. Today breakdown material falling into cave ponds ensures further enlargement of the dome like passages.

The water flow in the underlying limestone seems to be directed toward the southeast as can be inferred from an increasing in total mean hardness and the decreasing PCO₂ from the Hübichsaal (Location 18) to the Rhumegrotte (Location 15) and the Pfeilersee (Location 13). This interpretation agrees with the regional dip direction of the Zechstein. Thus, the PCO₂ in these pools in a function of the rapidity with which water can sink to the lower aquifer and of the spacing of falts, i. e. of the number of opportunities for the water to upwell into the level of the gypsum karst.

The Marthahöhle (Location 22) and the Klinkerbrunnen (Location 1 and 28) seem to depend on lateral inflow of water as well. The degassed carbonate water of the Bollerkopfbach enters the Marthahöhle system sideways. Both caves have higher mean gypsum saturation than the pools of the Jettenhöhle. On the other hand, their mean PCO₂ is much higher than that of the sinking creeks suggesting that these two caves derive their water from a mixture of low PCO₂ sink water and very high PCO₂ carbonate water from underlying limestone aquifers.

Group D
This Group comprises 8 stations from surface creeks and one gypsum karst spring (Location 20, Schurfquelle). The PCO₂ range from 2500 ppm to 980 ppm with the exception of Location 9 which shows a mean of 3800 ppm.

Location 9 represents the total discharge leaving the gypsum area toward the North and toward the Hackenbach (see hydrography on Map 3). This location combines the very low PCO₂ Schurfbach (Location 6) and the discharge from the high PCO₂ Beiersteinteich (Location 3) and the spring of location 8, even higher in PCO₂. In addition, a few meters above the sampling location 9, another spring enters the creek at its bed which appears to be high in PCO₂. Thus the location 9 does not show the low PCO₂ typical for the other surface creeks.

Obviously a water course of a few hundred meters is sufficient to effectively lower the PCO₂ of spring waters. This is best illustrated by the Schurfbach (Location 6). This creek is fed by three individual springs: the Schurfquelle (Location 20,1600 ppm), the 14-a-Quelle (Location 21,5300 ppm) and the Birkenbachquelle (2100 ppm). Once these springs have joined their waters, just 600 meters of swift turbulent flow in a shallow creek (a few centimeters in depth, 20-30 cm wide) will de gas the water to below a PCO₂ of 1000 ppm.

Also the Bollerkopfbachschwinde (Location 23) has a mean PCO₂ of 2500 ppm even though it is fed by a spring from the Staßfurt-Dolomite only two hundred meters upslope, which in 1973 had a PCO2 of - 6000 ppm (BRANDT et al., 1976).

The same phenomenon is shown by the Jettenbach (Location 24) which takes its waters from the Jettenquelle (Location 19) of 4500 ppm PCO₂ and the small Karpfenquelle (Location 26) with 14000 ppm PCO₂. After only 600 m of flow the pressure is down to 1800 ppm. The Heiligentalbach-South creek seems to be a reappearing surface water (Location 5, 1350 ppm), which delivers water collected by field drainage pipes further upslope of the dry valley of the Heiligental.

These examples are helpful for the later discussion on the origin of the high PCO₂ in large streams which have generally higher PCO₂ compared to these creeks.

Group E
This group contains three sample locations of seepage water from the Jettenhöhle. The thickness of the gypsum roof is roughly 15 to 10 meter. Sample 12 has the lowest CO₂ pressure of 730 ppm from all locations. The impact of the water jet on the rock floor increases sufficiently the water surface to expell all CO₂. Furthermore, this sample has the highest sulphate concentration encountered. The dissolution of gypsum occurs so rapidly, that water percolating through only a dozen meters of gypsum becomes saturated. Seepage water is therefore not responsible for the formation of cavities at the water table.

In winter after snow melt, the Jettenhöhle is flooded partly with seepage water. Water extruding from the ceiling has much lower PCO₂ as one would expect from the soil air PCO₂ of the wood covered surface (see Location 30). Water dripping from the ceiling and releasing calcite due to slow degassing (Kristallquelle) was found to have an overall CO₂ pressure of 2700 ppm.

All Hainholz locations react quickly to precipitation since their water storage capacity is small. They show an irregular pattern in all hydrochemical parameters in the course of the year. To determine PCO₂ variations sampling would have to be conducted on a daily basis. However, most of the PCO₂ time-series show quite a distinct seasonal signal. Some of these time-series are depicted in Fig. 5.