5 More Things to Know About USVI Hydrology

Do you know if streams in the USVI are properly referred to as guts or ghuts? Do you know where groundwater is available, and not available? Or are you curious about the ‘must read’ papers on USVI hydrology? My last article, 5 Things to Know About USVI Hydrology, received a lot of responses so I’m continuing the list. Enjoy.

1)????Are streams in the USVI referred to as “guts” or “ghuts”?

In the U.S. Virgin Islands, the local term for a stream is a “gut” or “ghut.” Both terms are pronounced 'gut' and both spellings exist in formal names of streams in the USVI. Authoritative, locally produced reports on the subject alternatively prefer “gut” (Gardner 2008a) and “ghut” (Gardner 2008b, Gardner, Henry and Thomas 2008). The shorter spelling “gut” is the most commonly used variation of the word, but use of the alternative “ghut” as a general reference to this geographic feature is common and acceptable.

How the terms came to be is a murky subject. The aforementioned Gardner et al. (2008) was unable to determine the origin of the terms. However, page 8 of that report does contain an entertaining and informative discussion of the likely origins as being related to the Indian term “ghat” and a Caribbean adaptation, “ghaut.” Many of us will recognize ghaut as a placename for streams in some parts of the Lesser Antilles, notably Montserrat and Saint Kitts and Nevis. It is possible that ghut and gut are simply further progressions of the term in the Caribbean. All of this as a long-winded way of saying we refer to streams as guts or ghuts, take your pick.

2)????There is groundwater… virtually everywhere

Groundwater is physically available throughout the entire USVI in three types of aquifers (water-saturated soil and geologic material). A volcanic rock aquifer underlies all of St. Thomas and St. John and most of St. Croix. The shallower portions of volcanic rock have weathered and developed many cracks, or fractures, that hold groundwater and allow it to flow. Wells drilled into this aquifer draw groundwater from these cracks and typically pump up to several hundred gallons per hour.

The Kingshill Aquifer is relatively large carbonate rock aquifer limited to central St. Croix and extending from north to south. This area is geologically separate from other parts of St. Croix. As a practical matter, well yields in the Kingshill aquifer can be relatively high, but are inconsistent and poor in large areas.

Alluvial aquifers consist of eroded material that is transported by moving water such as guts. This material settled in or near gut streambeds, especially where guts become less steep near the coast. Thus, alluvial aquifers lie on top of the fractured volcanic rock and Kingshill aquifers. Alluvial aquifers can often have yields considerably higher than either the fractured volcanic rock or Kingshill aquifers. However, alluvial aquifers that exist near the coast are vulnerable to saltwater intrusion.

All of this is to say groundwater is physically available throughout the USVI. Well yields are modest to poor, but typically provide at least many hundreds of gallons per day.

3)????Groundwater in the USVI has quality problems

Groundwater may be found everywhere in the USVI, but it is usually of poor quality. Groundwater throughout the USVI is usually high enough in dissolved substances to exceed EPA’s National Secondary Drinking Water Standard for non-hazardous, but nonetheless undesirable, substances. High amounts of dissolved substances in USVI groundwater makes the water unpalatable for drinking. However, this water can be treated to remove the offensive dissolved minerals or it may be used without treatment for non-drinking domestic uses. Managing highly-mineralized groundwater for irrigating crops is a complex issue that remains unresolved. ?

Another problem facing groundwater in the USVI is widespread microbial contamination. Half of USVI households depend on septic systems to treat household sewage. Many, or most, of these septic systems cannot function properly in the thin soils and steep terrain common in the territory and discharge poorly treated wastewater to groundwater. The other half of USVI households depend on a sanitary sewer system that is well beyond its expected lifespan and is prone to leak untreated wastewater from collection pipes and is prone to backup during wet weather. A pair of studies in the 80’s documented widespread contamination of groundwater with fecal coliform. Treatment of well water to remove microbial contamination prior to consumption is a wise practice.

In the end, groundwater in the USVI has water quality problems that can be overcome with household or farm-scale water treatment.

4)????Why does drought take a long time to develop and to resolve?

The ground has an amazing capacity to absorb water in times of excess and store it for use during times of drought. Globally, about 25 times more water exists in the ground than all of the lakes, rivers, and wetlands combined. Even within the thin soils and underlying fractured volcanic rock of the USVI, a large amount of water exists under our feet.

During drought, water stored in the ground is drawn down to support wells, plants, wetlands, and at one time, guts. But eventually, groundwater becomes depleted and can’t supply sufficient water as it once did. At this point, well levels fall dramatically or go dry, plants are unable to draw water from the ground and turn brown, and wetlands and guts dry out.

It is difficult to appreciate how much groundwater is lost from storage during a drought. And how much rain it takes to replenish groundwater to its former level. For groundwater to recover to its original level, water must soak into the ground, somehow avoid being captured by dense, parched vegetation, and percolate down to the water table. Even under normal conditions this is difficult, but following a drought it can be especially challenging.

Small rains, even if frequent, have little or no chance to recharge groundwater. A common estimate in decades-old USGS reports is that it takes about two inches of rain in a single storm to start recharging groundwater in the USVI. Only then can the storage of water in the ground begin to be replenished. This is a big part of the reason why it takes a long time to recover from drought in the USVI

5)????What are the ‘must read’ publications to learn about USVI hydrology?

Many dozens of papers and reports provide insight to USVI hydrology and each provides important information on how water flows through our islands. The following is an abbreviated list of publications to get the interested and ambitious reader started toward a more full understanding of USVI hydrology and water resources. Most or all of these publications are freely available online using a general web search engine or through Google Scholar. Full bibliographies are provided further below.

1970’s USGS reports on USVI water resources

USGS published a series of four reports that documented water resources on each main island in the USVI, as well as the effects of land use on hydrology in the territory. These reports are an excellent place to start learning the intricacies of USVI hydrology. See Cosner (1972), Jordan (1972), Jordan and Cosner (1973), and Jordan (1975).

USVI hydrogeology

Much of what we know of USVI hydrogeology is from a mid-1900’s doctoral research study done by Thomas Donnelly. Donnelly’s PhD dissertation is apparently unavailable (Donnelly 1959), but is summarized as a highly detailed geologic map of St. Thomas and St. John, available online from the USGS (Donnelly 1966). The 1984 USGS National Water Summary provides an approachable description of USVI groundwater resources, see U.S. Geological Survey (1985), especially pages 409-413. USGS groundwater atlases from the 1990’s provide a convenient, if brief, summary of the hydrogeology of the territory, including St. Croix. See Veve and Taggart (1996) and Miller et al. (1999).

Around the turn of the centry, two reports provided additional details regarding USVI geology. Rankin (2002) contains greater detail, but is limited to St. John. Renken et al. (2002) describes the geology throughout the territory, including an insightful characterization of the Kingshill limestone deposit and aquifer on St. Croix. Davis (2000) provides the results of an NRCS soil survey of the territory.

USVI runoff and flooding

A trio of 2008 reports from Lloyd Gardner describe the many guts (streams) in the USVI and their hydrology. See Gardner (2008a), Gardner (2008b), and Gardner et al. (2008). Reiblich and Ankersen (2016) summarize our understanding of gut hydrology and describe the regulation and management of guts in the USVI. A pair of papers by Carlos Ramos-Scharrón describe the impacts of roadbuilding on surface water runoff. See Ramos-Scharrón and LaFevor (2016) and Ramos‐Scharrón and LaFevor (2018). Following Hurricanes Irma and Maria, FEMA commissioned an H&H study (Hydrology & Hydraulics) for the USVI. The resulting report provides new ABFE flood zone maps of the territory and provides insight to hydrologic data availability, or the lack thereof, in the USVI. See STARR II (2018).

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Water quality in the USVI

A series of reports and papers describe the microbial and chemical water quality of waters throughout the USVI. See Garcia and Canoy (1984), Zack, Rodriguez-Alonso and Roman-Mas (1988), Cadmus Group (2011), and Jiang et al. (2020).

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Watershed-specific studies

There has been a proliferation of detailed hydrologic studies in the USVI in recent years. These reports are generally highly detailed and are often H&H modeling studies. See CDR Maguire Inc. (2015), Coral Bay Community Council (2015a), Coral Bay Community Council (2015b), Parsons Brinckerhoff (2015), and Stanley Consultants (2015).

Sources Cited

Cadmus Group. 2011. Watershed Characterization and Planning for Pathogen Source Reduction in the U.S. Virgin Islands. In U.S. EPA Contract Number EP-C-08-002, Cadmus Task Order 17.

CDR Maguire Inc. 2015. Gallows Bay Marine Terminal Area Drainage Study. Virgin Islands Port Authority.

Coral Bay Community Council. 2015a. Coral Bay potable water supply plan. Coral Bay Community Council, www.coralbaycommunitycouncil.org.

---. 2015b. Coral Bay Wastewater Treatment Plan. Coral Bay Community Council, www.coralbaycommunitycouncil.org.

Cosner, O. J. 1972. Water in St. John, US Virgin Islands. U.S. Geological Survey, Caribbean District, Open-file Report.Davis, J. R. 2000. Soil Survey of the United States Virgin Islands. United States Department of Agriculture, Natural Resources Conservation Service, 221 p.

Donnelly, T. W. 1959. Geology of St. Thomas and St. John, Virgin Islands. 179. Princeton University.

Donnelly, T. W. 1966. Geology of St. Thomas and St. John, US Virgin Islands. In Caribbean Geological Investigations, 85-176. Geol. Soc. Am. Mem.

Garcia, R. & M. Canoy. 1984. Reconnaissance of ground-water quality in the US Virgin Islands, July 1984. US Geological Survey.

Gardner, L. 2008a. Changes in riverine hydrology on St. Thomas, US Virgin Islands: A pilot study. Virgin Islands Water Resources Research Institute, University of the Virgin Islands.

---. 2008b. A Strategy for Management of Ghuts in the US Virgin Islands. In Water Resources Research Institute, University of the Virgin Islands. U.S. Virgin Islands: Water Resources Research Institute University of the Virgin Islands.

Gardner, L., S. Henry & T. Thomas. 2008. Watercourses as landscapes in the US Virgin Islands: state of knowledge. U.S. Virgin Islands: Water Resources Research Institute, University of the Virgin Islands.

Jiang, S. C., M. Han, S. Chandrasekaran, Y. Fang & C. A. Kellogg (2020) Assessing the water quality impacts of two Category-5 hurricanes on St. Thomas, Virgin Islands. Water research, 171, 115440.

Jordan, D. 1972. Land-Use Effect on the Water Regimen of the US Virgin Islands. U.S. Geological Survey Professional Paper 800-D, p. D211-D216.

Jordan, D. & O. J. Cosner. 1973. A survey of the water resources of St. Thomas, Virgin Islands. US Geological Survey.

Jordan, D. G. 1975. A survey of the water resources of St. Croix, Virgin Islands. US Geological Survey Reston, VA.

Miller, J. A., R. Whitehead, D. S. Oki, S. B. Gingerich & P. G. Olcott. 1999. Ground Water Atlas of the United States: Segment 13, Alaska, Hawaii, Puerto Rico, and the US Virgin Islands. US Geological Survey.

Parsons Brinckerhoff. 2015. Improvements to Veterans Drive (Route 30) - Stormwater Management Report Phase 1 & 2 Final Submittal. Miami, Florida: Government of The Virgin Islands, Department of Public Works.

Ramos-Scharrón, C. E. & M. C. LaFevor (2016) The role of unpaved roads as active source areas of precipitation excess in small watersheds drained by ephemeral streams in the Northeastern Caribbean. Journal of Hydrology, 533, 168-179.

Ramos‐Scharrón, C. & M. LaFevor (2018) Effects of forest roads on runoff initiation in low‐order ephemeral streams. Water Resources Research, 54, 8613-8631.

Rankin, D. W. 2002. Geology of St. John, US Virgin Islands. US Geological Survey.

Reiblich, J. & T. Ankersen (2016) Got Guts: The Iconic Streams of the US Virgin Islands and the Law's Ephemeral Edge. J. Envtl. L. & Litig., 32, 71.

Renken, R. A., W. Ward, I. Gill, F. Gómez-Gómez & J. Rodríguez-Martínez. 2002. Geology and hydrogeology of the Caribbean islands aquifer system of the commonwealth of Puerto Rico and the US Virgin Islands. U.S. Geological Survey.

Stanley Consultants. 2015. Spring Gut bridge replacement H&H study.

STARR II. 2018. US Virgin Islands Advisory Data and Products Post-Hurricanes Irma and Maria. Federal Emergency Management Agency (FEMA).

U.S. Geological Survey. 1985. National Water Summary 1984—Hydrologic events, selected water‐quality trends, and ground‐water resources. In US Geological Survey Water-Supply Paper 2275, 467.

Veve, T. D. & B. E. Taggart. 1996. Atlas of ground-water resources in Puerto Rico and the US Virgin Islands. In Water-Resources Investigations Report 94-4198, https://doi.org/10.3133/wri944198. San Juan, Puerto Rico: U.S. Geological Survey.

Zack, A., T. Rodriguez-Alonso & A. Roman-Mas (1988) US Virgin Islands ground-water quality. US Geological Survey Open-File Report, 87, 0756.