Client Name: Proyecto Yaxchen, Kay & Gary Walten

Sampling Site: Yaxchen Cave System

Sampling Date: June 3, 1998

Discussion of Results

pH

The pH of most natural waters is between 4 and 9, usually between 6 and 8. In water, a deviation from neutral 7.0 is normally the result of the hydrolysis of salts and strong bases and weak acids or of weak bases and strong acids. However, gases such as CO2, H2S and NH3 can have a significant effect.

In Yaxchen, all sites have a pH between 7.3 and 7.6. The water is alkaline (slightly basic) in this region because of the presence of carbonate and bicarbonate found in the limestone of which this cave system is made. Carbonate and bicarbonate prevent acidification.

Salinity

Salinity is a measure of salts in the water, and its units are often presented in parts per thousand, or ppt. Seawater has a salinity of 35 ppt. Where fresh and salt water meet, the resulting mixture is called brackish water, defined as having a salinity between 0.5 ppt and that of full-strength seawater. The salinity of the sites sampled in Yaxchen range from 8 to 10 ppt. These numbers indicate that the water is, indeed, brackish. 3 of the 5 sites have a salinity of 8 ppt, one has a salinity of 8.5, and the cloudy layer located below the halocline has a salinity of 10 ppt. The cloudy layer is more saline than its surrounding waters. It would be interesting to get salinity readings below the cloudy layer itself as this, too, should be saltier water.

Fecal Coliform

Fecal coliform are an indicator species of other pathogens that may be in the water and which, in turn, may pose a threat to human health. Fecal coliform are present in the Yaxchen cave system. The numbers achieved (i.e. 6-16 colonies/100 ml) is well below the 200-colonies/100 ml limit for safe body contact. These numbers do, however exceed the 0 colonies/100 ml limit for safe drinking water. We can dive it, but we cannot ingest it.

Nitrogen Cycle Nitrogen can exist in various forms in the aquatic environment. The greatest source of nitrogen on our planet is air. This nitrogen, however, is only available to those aquatic organisms that can fix it into a form available to the rest of the biota. This 'fixing' is performed by only a few species of bacteria and blue-green algae.

The nitrogen cycle is a 'double cycle'…one side of the cycle involves oxidation and reduction of nitrogen by plants, animals and decomposers, while the other entails nitrogen-fixing organisms and denitrifying bacteria (Lind, 1985).

Ammonia is the most reduced form and is the product of organic decomposition. The oxidized forms of ammonia are nitrites and nitrates. Nitrite and nitrate, on the other hand, result from the nitrification (bacterial oxidation) of ammonia.

Denitrification, the bacterial reduction of nitrate to nitrite and then to N2 gas, occurs at low oxygen levels.

Ammonia

If streams are diverted through wetlands (e.g. mangrove), it is possible to remove NO3 by denitrification. NH4 is the preferred form of nitrogen for plant growth.

Ammonia is the product of organic decomposition.

Ammonia is usually present in low (less than 1 mg/L) concentrations in non-polluted, well oxygenated water, but may reach up to 5-10 mg/L in the anaerobic hypolimnion of a eutrophic lake (Lind, 1985).

The ammonia levels found in Yaxchen ranged from 0.00-0.24 mg/l NH3-N. These levels are low and indicate their source is non-polluted and well oxygenated. All of the ammonia levels were 0 mg/l NH3-N throughout the system, except in an area known as the crackline. Here, a cloudy layer persists, just below the halocline. This is where NH3-N reaches 0.24 mg/l. Above this cloudy layer, in the freshwater zone, another sample was taken. Its reading was 0.02 mg/l NH3-N. Although the levels found in the cloudy layer are low, relatively speaking, the presence of ammonia in this area suggests a zone low in oxygen (in comparison with the other water there). Ammonia can only be converted to nitrite or nitrate in the presence of oxygen. And in the absence, nitrate is converted to nitrite and nitrite is converted to ammonia.

These findings also suggest that there may be denitrifying bacteria present, which are present only in the cloudy layer which convert nitrates to nitrites and ammonia. The cloudy layer may be the bi-product of their metabolism.

Nitrites

Nitrite (NO2--N) is the partially reduced form of nitrate (NO3--N). It is the intermediate state between nitrate and ammonia. Nitrites in the Yaxchen system are highest in the cloudy layer found below the halocline, although the difference between this site and the others is not substantial. The nitrite concentrations in Yaxchen, like they are in most systems, are low.

Nitrates

Nitrate nitrogen is usually present in low concentrations in natural water. Natural concentrations generally don't exceed 10 mg N/l and are commonly less than 1 mg/l, especially during periods of increased primary production (i.e. algal photosynthesis).

In Yaxchen, an interesting story prevails with respect to the nitrogen cycle. During the first run of samples, I did a low range nitrogen test. Only one sample was within this range: the 'cloudy layer'. Its concentration is 0.18 mg/l NO3--N. All other sites were above the range of the test. The high range nitrate nitrogen test provided the appropriate range for the remainder of the samples, whose concentrations ranged from 0.8-1.2 mg/l, with an average of 1.0 mg/l NO3-N. This means that the surrounding water has almost 6 times the amount of nitrate than the cloudy layer!

Thus, it appears, from the high ammonia levels and low nitrate levels in this cloudy layer, that denitrification is taking place; nitrate is being converted to ammonia. This process might be occurring because of a lack of oxygen in this cloudy layer and/or denitrifying bacteria may be present here.

Total Phosphorus

Total phosphorus concentrations of non-polluted waters are usually less than 0.1 mg P/L. The average P concentration in Yaxchen is 0.112 mg/l, with the cloudy layer having the highest concentration at 0.25 mg/l. The average concentration of the four remaining sites is 0.008 mg/l.

At normal lake pH ranges, most soluble phosphate is present as orthophosphate in 2 ionic forms: monophosphate (HPO42-) and dihydrogen (H2PO4-) phosphate ions. Changes between these forms occur rapidly as pH changes. Since the pH in Yaxchen is essentially constant, these changes are unlikely to occur.

Phosphate ions (PO43-) are adsorbed or desorbed from particles depending on the external phosphate concentration and salinity. PO4 is more readily released in water with higher salinity. More than likely, this is what is happening in the 'cloudy layer', where the salinity is higher than the surrounding water.

Turbidity

Turbidity in this system is low, with values ranging from 0 to 7.3 FAU. The greatest turbidity is found within the cloudy layer, as expected.