Preliminary Hydrochemical Interpretation of Carroll Cave

The interactions of rainwater and groundwater with the soils and rocks in which they flow are often complex and difficult to interpret.  It is my intent to present the raw hydrochemical data collected in Carroll and Toronto Spring, provide a basis of interpretation, and then discuss several possible conclusions that the data suggest. There are some “mysteries” that the data uncovers, but also some interesting ideas on cave genesis are suggested by the data.  One possible impact on the biology of the cave is found and will be discussed.

First, how does a cave begin to form in most carbonate terrains?

The attribute of solutions, pH, refers to the acidity of the solution. This factor is measured on a logarithmic scale like the Richter seismic scale:  a solution with a pH of 7 is 10 tines as acidic as a solution with a pH of  8. Distilled water without mineral constituents has a pH of 7 and is considered chemically neutral. Rainwater is naturally acidic with a pH in the range of 5-6 (except in industrial areas where the pH of the water can be lower).

As rainwater falls through the atmosphere, it combines with carbon dioxide to form a weak acid:

H20 + CO2 ® H2CO3 or carbonic acid

As this weak acid enters the soil zone, it often increases in concentration due to the interaction of decaying plant material and oxygen in the soil:

O2(gas) + CH20(organic material) ®  CO2+ H20

Carbon dioxide and evolved water from this reaction then can combine:

                              H20  + CO2® H2C03 (carbonic acid)

So two basic chemical reactions related to rainfall and soil decay form a near-surface aqueous solution low in pH that is ready to immediately alter both soil water and groundwater chemistry.

As this solution infiltrates from the soil zone, it first encounters either the Roubidoux or Gasconade formation in the area around Carroll Cave.  Both formations contain dolomite and to a lesser amount calcite.

Dolomite is a carbonate mineral that contains both calcium and magnesium:

Dolomite         Ca++ + Mg ++   + 2CO3- -  ® CaMg(CO3)2

Calcite contains only calcium as the cation or positively charged chemical species:

Calcite             Ca++ + CO3- -  ®    CaCO3

The amount (concentration) of either of these minerals that can be dissolved in water is highly dependent on temperature, presence of dissolved CO2 in the percolating water, and the pH or acidity of the solution. Calcite and dolomite are unusual minerals in that they become easier to dissolve under lower temperatures.  Most all other minerals have higher solubility  (are easier to dissolve) at higher temperatures.  This phenomenon is because the amount of CO2 that can be dissolved in an aqueous solution increases with decreasing temperature.  As the amount of C02 in a solution increases, the capacity of the solution to dissolve dolomite or calcite increases. Conversely, as temperature rises or the amount of dissolved CO2 decreases, the ability of the solution to dissolve dolomite or calcite decreases and the solution trends toward equilibrium or saturation of that mineral. Also, the pH of the solution helps control the solubility of a mineral.  For calcite and dolomite, as pH lowers (becomes more acidic), the ability for a given solution to dissolve these minerals increases.  The ideal solution to dissolve dolomite or calcite is a cold, highly acidic (low pH) solution,  that contains no dissolved minerals.

 Saturation is when 100% of a particular mineral that can be dissolved into a solution for a given set of pH, temperature, dissolved CO2, and associated mineral concentrations conditions is dissolved into that solution without precipitation of that mineral as a solid phase.  Supersaturation (>100%) indicates that a mineral is past saturation and can precipitate out of solution as a solid. Undersaturation indicates that the solution is capable of dissolving that mineral.

Concentrations of a mineral or related chemical species in a solution alone can’t describe the saturation condition of a mineral.  If temperature, pH, dissolved concentrations of associated minerals and CO2 content of the solution remain relatively constant, then an indication of the ability of a specific mineral to be dissolved or precipitated can be estimated from concentration data alone.  The preferred method of investigation takes into account as many relevant factors as possible in determining the saturation condition of a particular mineral.  In the Carroll area, pH of the solution has the dominant impact on solubility of calcite and dolomite, since temperature and concentration of individual chemical species changes little.  For example, temperature in the cave and spring were measured to vary from 57.3 to 58 degrees (1.2% variation).  The concentration of the major anion chemical species, bicarbonate, varied from 251 to 264 parts per million (ppm) from the entrance shaft to Toronto Spring (4.9% variation) yet there is a significant change in the saturation states of calcite and dolomite between the entrance shaft and Toronto Spring.  This saturation variation is caused by a change in the pH of the ground waters from 8.06 pH at the entrance shaft to 7.2 pH at the spring (724% variation).

Most natural groundwater, given enough time, trend towards equilibrium or saturation for each constituent mineral, which composes the rock in which the groundwater flows.  In other words, as a groundwater solution encounters dolomite and limestone, it quickly reaches saturation because of the quick reaction time of:

H2C03 (carbonic acid) + CaCO3 (calcite) ® Ca++(calcium cation)  + 2HCO3-(bicarbonate anion) and/or:

                        2H20 + 2C02 ® 2H2C03 (carbonic acid) , then

2H2C03 (carbonic acid) + CaMg(CO3)2 (dolomite) ® Ca++(calcium cation)  +  Mg++(magnesium anion) + 4HCO3-(bicarbonate anion)

So in ground waters that are dissolving dolomite, the Ca++(calcium cation), Mg++(magnesium cation) and HCO3- (bicarbonate anion) are the dominant, major chemical species.

If there is an abundance of CO2 in the soil to help maintain the concentration of H2C03 in groundwater, then calcite and dolomite are quickly dissolved and the reaction trends towards equilibrium or saturation of each mineral.  Generalities cannot be made in carbonate systems which have both calcite and dolomite as to which mineral will dissolve first (i.e., calcite will dissolve and equilibrate first and then dolomite) because of the impacts of the sequence of which mineral the solution encounters and the other considerations of pH, abundance of carbonic acid (CO2), and temperature.  In most carbonate systems, as soil water enters the vadose zone,  it quickly is saturated with respect to calcite and dolomite. As the solution reaches the water table, the water chemistry can either remain at saturation or become unsaturated. It is this change from saturation to undersaturation for calcite and dolomite that we see going from Upper Thunder River to Toronto Spring.    (Reference- Freeze and Cherry, GROUNDWATER, 1979.)

Hydrochemical Data and Data Display - Concentration Data

The following are the locations of the water samples:

Sample Locations – Red Triangles  & Water-Table Elevations and Flow Directions

Samples were collected in inert plastic containers on Sept. 7, 2002.  The samples were sent for analysis on Oct. 5, 2002, completed for analysis on Oct. 11, 2002 and the results of analysis received back on Oct. 14, 2002. All samples were analyzed by the Soil and Plant Testing Lab on the campus of the University of Missouri –Columbia, except for the sample at Toronto Spring which was collected by Mr. Jerry Vineyard for his publication of “Springs of Missouri.”

The following is the set on analysis performed on the water samples: (na-not analyzed)

Tests 1-18 were conducted by the lab and analysis 19-32 were done by Hall.

As indication of the accuracy of the analysis, a charge balance error for each sample was made using WATEQ (USGS geochemical modeling program).  Charge errors were well under 10%, indicating the accuracy of each analysis (#32 in table above). In order to better interpret the lab results, maps and x-sections were constructed to show the relationship of sample locations to the particular chemical species being investigated.

The maps are arranged in order of decreasing cation or anion concentration, with arrows pointing in the direction of decreasing concentration of that particular chemical species.

For this study, major cations/anions are over 10 ppm, minor cations/anions are between 10 and 1 ppm, and trace cations/anions are less than 1ppm.

Note that entrance shaft data not included in mapped data, but is included on hydrochemical x-sections to be displayed later.

The following are maps of the major cations in the water samples and their locations:          

Calcium Concentration (ppm)                                  Magnesium Concentration (ppm)

The following are maps of the minor cations in the water samples and their locations:

Sodium Concentration (ppm)                    Potassium Concentration (ppm)

The following are maps of the trace cations in the water samples:

  Iron Concentration (ppm)                Manganese Concentration (ppm)

                                    Copper Concentration (ppm)

The following is the map of the major anion and sample locations:

Bicarbonate Concentration (ppm)

The following are the minor anions and sample locations:

    Sulphate Concentration (ppm)         Nitrate Concentration (ppm)                  

Carbonate Concentration (ppm)          Chloride Concentration (ppm)

The following is the trace anion and sample locations:

   Phosphate Concentration (ppm)

This table summarizes the sample concentration data:

> 10 ppm                                Major Cation- Calcium, Magnesium                      Major Anion- Bicarbonate

 <10ppm - >1ppm                  Minor Cations- Sodium, Potassium        Minor Anions-Sulphate, Nitrate, Carbonate, Chloride <1ppm                    Trace Cations- Iron, Manganese, Copper     Trace Anion- Phosphate

The followi