Mustang Island Aquifer
This Earthcache is at Joan & Scott Holt Paradise Pond, Port Aransas. Since this is an Earthcache, there is no physical container to find. To get your smiley for the cache, e-mail or message your answers to the questions below to the cache owner BEFORE you log a found it.
Mustang Island
Mustang Island is 18 miles long and varies from 3,000 feet wide on the south end to just over two miles wide on the north end. It is part of a barrier island chain along the Texas Gulf Coast. Barrier islands protect the bays and the mainland from the Gulf of Mexico's waves, wind, tides, and storms.
How were these islands formed? Around 18,000 years ago, as the most recent ice age waned, the sea level was lower than it is now. Rivers carried sand and soil to the gulf, creating sandbars.
Over time, the sea level rose, and the ocean currents carried sand from those sandbars toward the present-day shore. The sand built up, eventually forming the long, skinny barrier islands about 2,500 years ago. These islands are almost entirely made of sand.
Barrier islands are dynamic landforms. This means they continue to change, sculpted by climate, wind, waves, tides, ocean currents, and storms. Humans have altered the islands, too.
Ground Water & Aquifers
A huge amount of water exists in the ground below your feet, and people worldwide use it. But it is only found in usable quantities in certain places underground — aquifers. Groundwater is one of the most valuable resources on Earth.
The saturated area beneath the water table is called an aquifer, and aquifers are huge storehouses of water. In our Joan & Scott Holt Paradise Pond example, you have essentially a "pond" that exposes the water table with an aquifer beneath it. At the beach, the water table level is always the same level as The Gulf of Mexico, which is just below the beach's surface.
The diagram below shows how the ground below the water table (the blue area) is saturated with water. The "unsaturated zone" above the water table (the gray area) still contains water (after all, plants' roots live in this area), but it is not totally saturated with water. You can see this in the two drawings at the bottom of the diagram, showing a close-up of how water is stored between underground rock particles.
Water movement in aquifers is highly dependent on the permeability of the aquifer material. Here on Mustang Island, sand is a permeable material that contains interconnected cracks or spaces that are numerous and large enough to allow water to move freely. In some permeable materials, groundwater may move several meters a day; in other places, it moves only a few centimeters in a century. Groundwater moves very slowly through relatively impermeable materials such as clay and shale.
Groundwater on Mustang Island
The Mustang Island aquifer is recharged only from rain falling directly on the Island and by surface runoff from that rain. The aquifer has a maximum thickness of between 75-150 feet. It is thickest in the vicinity of the dune line and thins to almost nothing at the boundary with Corpus Christi Bay.
The source of all of the fresh groundwater on Mustang Island is rain and surface runoff from rain sinking into the ground. The diagram below of the water cycle shows how the groundwater is replenished. The portion of the rain which does not evaporate, runoff, or return to the atmosphere by transpiration by plants, flows down into the sand of the Mustang Island
aquifer and slowly flows into the sea, into the ship channel, and into Corpus Christi Bay.
Coastal aquifers in isolated sand bodies surrounded by seawater, such as Mustang Island, are special because freshwater is floating on top of the sand body. In the case of our Mustang Island aquifer, the freshwater is only a tiny bit lighter than the surrounding salt water, so it barely floats on it.
Seasonal Differences
Due to the hydrologic cycle, seasonal differences occur at the Joan & Scott Hold, Paradise Pond. Although it may appear that during winter and summer when the water levels are low or nonexistent, the Pond or Wetland is “dead.” However, it is crucial for the water levels to lower so that the vegetation can germinate or drop seeds. Additionally, when water levels are high in the fall and spring, it creates a beneficial environment for residents, migratory birds, and other wildlife.
Seawater Intrusion
There are two ways the Mustang Island Aquifer can be degraded by seawater intrusion. Flooding of the low-lying areas of the island during hurricanes can cause a direct inflow of salt water down from the surface. Secondly, over-pumping of individual wells can cause localized seawater intrusion from below.
LOGGING REQUIREMENTS:
Please email us the answers to the following questions, DO NOT POST the answers in your log-
1) What two seasons are the lowest water level at Joan & Scott Holt, Paradise Pond?
2) What are the two ways that Mustang Island Aquifer can be recharged?
3) At wave point one, estimate the distance between the floor (top) of the boardwalk and the top of the water. Explain why you think the pond is at this level.
4) Looking across the pond, estimate the distance across the open pond – no cattails growing.
5) What do you think would happen to Mustang Island Aquifer if the aquifer's freshwater lens is breached by drilling?
6) Please post a picture (Optional) of you and your GPSr at the posted coordinates to show your visit or of you and Paradise Pond.
PLEASE NOTE: I will not be sending follow-up requests for answers. If you do not complete the requirements, your log will be deleted. To claim this Earth Cache, you must follow through with the requirements, not just visit the site.
Permission for this Earthcache was granted by Colleen Simpson, Director of Port Aransas Park & Recreation.
References:
https://www.usgs.gov/special-topics/water-science-school/science/aquifers-and-groundwater
http://texascoastgeology.com/papers/pa_water.pdf
Brown, L.F., Jr., Morton, R.A., McGowen, J.H., Kreitler, C.W., and Fisher, W.L. 1974. Natural Hazards of the Texas Coastal Zone, Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas.
Seawater Intrusion in Coastal Aquifers, Guidelines for study, monitoring, and Control. 1997, Food and Agriculture Organization of the United Nations, Rome 1997.