Groundwater, haloclines and water quality in flooded caves

An underwater cave is not a closed room full of still water. It is part of an aquifer, the body of permeable rock that stores and moves groundwater, and the water a diver swims through is usually on its way somewhere: to a spring, a river, a well or the sea. That is why cave divers often see the same water quality problems that hydrologists measure in wells, and why their observations are useful to science.

Karst aquifers and conduit flow

In karst, water moves through three kinds of space at once: tiny pores in the rock, enlarged fractures and vugs, and open conduits large enough to swim through. The US Geological Survey (USGS) describes the Floridan aquifer system as a thick sequence of Tertiary carbonate rocks covering about 260,000 km² (100,000 square miles) of the south-eastern United States, and as the main source of drinking water for almost 10 million people ^[2]. Florida's Department of Environmental Protection (DEP) states that groundwater supplies about 90% of Floridians' drinking water and calls springs a window into the health of that groundwater ^[11].

The Yucatán Peninsula has an even larger karst aquifer. A 2011 review in Hydrogeology Journal describes a transboundary system of about 165,000 km² across Mexico, Guatemala and Belize, with no surface rivers in much of the region, seawater intruding tens of kilometres inland and usable fresh water restricted to a thin lens, less than 10 to 100 m thick, floating on salt water ^[1]. The review notes that the world's largest underwater cave systems are embedded in this aquifer ^[1].

Conduits make karst aquifers fast. The Wakulla 2 project's geology summary describes how, in the Woodville Karst Plain south of Tallahassee, water enters through sinkholes and sinking streams, and how hydrologists found a groundwater divide 1 to 2 km inside Wakulla's main tunnel, with water north of it flowing to the spring and water south of it apparently heading towards submarine springs ^[9]. The same summary records that in September 1999 much of Lake Jackson, a lake of about 1,600 hectares, drained down a single sinkhole in a day ^[9]. Events like these show how directly surface water can reach the caves.

Haloclines and anchialine water

Near coasts, fresh groundwater floats on denser seawater that has seeped in from the sea. The boundary is the halocline, a zone where salinity changes sharply, often with a change in temperature as well. Divers see it as a shimmering, blurry layer where the two waters mix.

The Quintana Roo Speleological Survey (QRSS) explains that Quintana Roo's caves typically carry an upper freshwater layer flowing over a nearly static saltwater intrusion. The halocline deepens and the freshwater layer thickens with distance from the sea. Above the mixing zone, chloride in the fresh water stays near about 2 parts per thousand, while below it salinity often jumps to between 14 and 35 parts per thousand depending on depth and distance inland ^[3]. At Hoyo Negro, about 7 km from the coast ^[5], the halocline lies at 15 to 22 m below sea level, and the aquifer level there follows sea level to within 1 to 2 m ^[4].

Water bodies like these, inland but connected underground to the sea and rising and falling with the tide, are called anchialine. A 1999 study of Bundera Sinkhole in Western Australia defined anchialine ecosystems as inland underground waters of mixed salinity, affected by marine tides, usually with little surface exposure ^[7]. Researchers working in Ox Bel Ha have described such systems as subterranean estuaries, because fresh, brackish and saline layers sit one above the other in the flooded passages much as they do where a river meets the sea ^[8].

Some blue holes add a third layer. In Sawmill Sink on Abaco, The Bahamas, a 2007 study recorded fresh water from the surface to about 9.5 m, a halocline zone from about 9.5 to 27 m in which hydrogen sulfide and sulfur-reducing bacteria block light, and anoxic marine water below about 27 m that follows the ocean tides with a lag of an hour or more ^[6]. Hydrogen sulfide layers like this are a known feature of some anchialine caves; the Bundera study also described a sulfidic layer near its thermohalocline ^[7].

Chemistry that supports life

These layers are more than a curiosity. A 2017 study in Ox Bel Ha found that methane and dissolved organic carbon from the overlying forest soil feed a community of microbes in the flooded caves, and that cave-adapted shrimp obtain on average about 21% of their diet from methane-consuming bacteria ^[12]. The USGS, a partner in the work, noted that methane formed in soils here migrates down into the water and caves instead of only up into the atmosphere ^[8]. The article on cave ecosystems explores what lives in these layers.

Visibility, tannins and floods

Divers judge water first by visibility, and in springs this is often a hydrological signal. In north Florida, rain washes tea-coloured tannic water from swamps and rivers into sinkholes and then through the caves. The Wakulla 2 summary reports that tannic and humic acids from surface waters are the main cause of reduced clarity at Wakulla Spring, that records showed poor visibility 58% of the time over a 12-year period, linked to rainfall, and that the clearest water tends to come in the dry months of May and June ^[9]. These are historical figures from a 1990s source; they illustrate a pattern rather than predict any particular day.

Contamination risks

The same openness that makes karst a rich water source also makes it vulnerable to pollution. The Bundera study summarised why: thin karst soils filter little, open conduits spread contaminants quickly and far, and in dry regions low groundwater flushing means pollutants can stay a long time ^[7].

In the Yucatán, the 2011 review identified wastewater as the main groundwater quality problem. With no rivers to discharge into, most wastewater is injected back into the aquifer or released into cenotes or lakes, and only about one third was treated before disposal ^[1]. Under Mérida, where the freshwater lens is only about 60 m thick, the review cites the national water commission as saying the upper 20 m is unfit to drink because of septic-tank pollution ^[1]. It also reports widespread nitrate in wells across Yucatán State, linked to agriculture and local wastewater, and warns that rapid tourism development on the Riviera Maya, with growing volumes of wastewater and solid waste and no comprehensive wastewater regulation, threatens both water supply and groundwater-dependent ecosystems; landfills are singled out as especially critical in such permeable rock ^[1].

In Florida, the DEP identifies two main threats to springs: falling flows as aquifer levels decline, and excess nutrients, mainly nitrate, that feed algae and degrade habitat. The legislature named 30 Outstanding Florida Springs in 2016 and in June 2018 the DEP adopted 13 basin management action plans covering all 24 of them that were impaired by nitrogen ^[11]. Cave divers have contributed directly to this picture: Global Underwater Explorers reported that Woodville Karst Plain Project research showed the caves carry water many kilometres across the region and that water from Tallahassee's sewage treatment facility was contaminating the groundwater, which it says helped prompt the city to refine its water management ^[10].

Why divers' observations matter

Divers record haloclines, flow, visibility and sometimes water samples deep inside aquifers that hydrologists otherwise see only through wells. That access is valuable, but it also places divers inside drinking-water supplies. Anything left in a cave, from waste to disturbed sediment, ends up in the same water. The articles on conservation and current threats look at how this is being managed.