Update May 2018
From 19 to 30 March 2018 another stay in Mexico / Tulum took place. During this time, a series of activities and events around the World Water Day 2018 were scheduled, mainly initiated / organized by our partner Amigos de Sian Ka'an in schools and public spaces. The small Citizen Science project was also presented on the occasion. As it turned out, the project's content and objectives fit perfectly into the growing diversity of local activities as an expression of increasing public attention for the groundwater issue in Tulum ...
What we measure and what we learn from it:
There are very different water parameters we measure. First of all let us focus on the most convenient test, the 'ProScan'-strip test with smartphone application from the german companyJBL.
Let us see what interesting questions can be dealt with this quick test.
The parameters measured with this test are nitrite NO2, nitrate NO3, general hardness GH, catbonate hardness KH, acidity pH, chlorine Cl2 (less important) and carbon dioxide CO2.
The questions we try to answer with these measurements are:
How is the cave and conduit system developing?
How active is the limestone dissolution and what are the main actors?
How is the connectivity and what are the main flow paths?
How is the activity of bacteria affecting the system?
How do cenotes form?
Within the context of these questions the test can already give us valuable information which is described a little bit more in the following:
Hardness of the water (GH, KH)
Hardness tells about the concentration of dissolved ions of alkaline earth metals. In the region of Tulum, the most important 'hardness-players' in the freshwater layer are calcium and magnesium from dissolution of limestone and dolomite, when water is flowing through cracks, fissures or underwater caves, or the mixing with saline water in the mixing zone around the halocline since the mixture has higher solution ability.
General or total hardness GH covers all ions in solution, dissolved by carbonic acid other acids (nitric or sulfuric). It is the sum of carbonate hardness KH, and non-carbonate hardness.
Carbonate hardness KH is actually the amount of calcium (Ca++) ions in the solution which are allocated to hydrocarbonate (HCO3-) ions. Under certain conditions, these ions combine and settle as limestone.
Non-carbonate hardness is the contribution of remaining Ca(2+) without a HCO3- and bicarbonate CO3(2-) partner and Mg-Ions dissolved by other than carbonic acid.
Why is hardness important? It is not only because it can produce coats of solid limescale in cooking pots, coffee makers or washing machines. For us, however, it is because it can tell us about the current 'activity' of the karst development, i.e. the development of the whole conduit and cave system in the Tulum subsurface.
This value gives the most basic information about the acidity of the water. Axayacatl Maqueda Estrada, doctoral student at the University of Neuchatel/ CHYN: 'Most pH values in water samples from Tulum are a little bit above or below 7 because water has been in contact with soluble carbonate rocks. Most fresh and saline samples are near equilibrium with calcite. One surprise would be if someone measures low pH, i.e. 6. This would indicate high levels of carbon dioxide originating from bacteria decomposing organic matter.'. The measurement of the pH-value helps to estimate carbon dioxide concentration.
The dissolution of the limestone in the ground water needs an actor and this is the dissolved carbon dioxide, which becomes an acid in the water - carbonic acid. Bacteria are emitting the carbon dioxide in the soil above the conduit systems or in the water as we do above the water when breathing. For staying alive, the bacteria need food, which comes from dead plants and other organic matter (fertilizer, fecal matter).
Therefore, the CO2 distribution gives a good picture where bacterial and dissolution activity is running high level, organic matter is circulating. However, the CO2 is not measured directly in this test but calculated from the carbonate hardness and the pH value.
If we could map the distribution of CO2 distribution with sufficient temporal and spatial coverage, we get some a picture about the limestone dissolution activity in the karst system, which is like watching where the cave system grows. And it is an open question: is bacteria/CO2 production distributed or localized in some points in Tulum?
Nitrite (NO2-) + Nitrate (NO3-)
These components generate during degradation of organic material by bacteria. Nitrite is poisonous. Its concentration is increasing if the input of organic material increases and/or the bacteria does not manage to transform sufficient Nitrite into Nitrate, or there is not enough oxygen for oxidizing it.
Nitrate is important food for plants, the basic provider of nitrogen. Increasing concentration causes most probably rapid growth of alga and shortage of oxygene in the presence of light. Through conduits Nitrate could be transported over large distances (eg. to the reef)
Thus, both, Nitrite and Nitrate, are pollutants depending on their concentration. Main concern is their presence in drinking water because of adverse effects on health: vitamin A deficiency, anemia, etc. children are more vulnerable (see article about nitrite/nitrate in groundwater in 'downloads').
The main nitrate sources in the area are untreated human wastewater, and pig farms. In 2009 it was estimated a pig population around 670 000 individuals. However, pigs have a faster metabolism (7x) than humans. Pigs generate more waste, thus 670 000 pigs are equivalent to 4.4 million people in terms of pollution. For reference, Yucatan human population is around 2 million. Basically, the waste of all farms goes untreated into groundwater (for more details see article in the 'Downloads' - 'impact of pig farms').
In conclusion, nitrite and nitrate measure human (and pigs) impact to groundwater.
To evaluate the complex process of limestone dissolution, i.e. the development of caves in the conduit system as best as possible, all the parameters have to be considered - GH, KH, CO2, pH, electrical conductivity and salinity with hydrometers (important!), as well as rainfall. Nitrite and Nitrate give information about the water quality.
Water level, temperature, salinity (via electrical conductivity and/or density measurements), flow velocity, as well as oxygen, phosphate, and e.coli would complete the monitoring and provide comprehensive data enabling different analysis and a very profound insight into this complex and versatile karst groundwater world.
An increase in the number of measurement sites and measurements is also sought, since with good spatial and temporal coverage of the measurement area, the distribution and variation of water quality and maybe even propagation paths for possible pollutants can be estimated.
For karst research, the best possible map of the lime-dissolving capacity is essential (derivable from the parameters GH, KH, Co2, pH, Ca, Mg).