Volcano Watch: Volcano Stratigraphy and Hydrology--layers and groundwater.

Reprinted from: http://hvo.wr.usgs.gov/volcanowatch/2009/09_07_30.html

A Volcano's Many Layers Determine the Richness of Hawai`i's Groundwater Resources

Fresh water cascades from a spring on a cliff face into a pool next to the Pacific Ocean near Nahiku on the island of Maui. The water comes from rain that fell at higher elevations on Haleakala Volcano, seeped into the ground, and has traveled slowly toward the coast. (USGS photograph by Gordon Tribble from http://pubs.usgs.gov/circ/1312/c1312.pdf)

Groundwater provides most of the fresh drinking water used in Hawai`i. Thus, discoveries that increase our understanding of the groundwater stored in our shield volcanoes are of great importance to us.

In matters of water, an island's size counts, because the ability to wring moisture from passing air masses and hold it as groundwater depends on an island's altitude and area above sea level. Also important is an island's volcanic history, because it determines the rocks' porosity (ability to absorb fluids) and permeability ("flow-through-ability"), which, in turn, affect the flow and storage of groundwater.

Having grown from the sea, a volcanic island is infused with seawater. Fresh water originates as rain, fog drip, or snow, some of which percolates into the lava flows to recharge groundwater. Most groundwater filters downward to sea level within the island, where it forms a freshwater lens-shaped body that floats upon denser salt water. The fresh-water lens thickens slightly inland, but, in most places, the top of the lens stands no more than a few meters above sea level.

The fresh-water lens is dynamic: groundwater flows constantly from inland areas of recharge to be discharged at the coast. Vertical dikes in rift zones tend to retard the lateral flow of groundwater and may impound (confine and store) it at high altitudes. Buried soil layers and ashy beds with low permeability impede the downward migration of water. They cause—above the fresh-water lens—the formation of small, perched aquifers, bodies of rock permeable enough to conduct groundwater. These dike-impounded and perched aquifers feed scattered springs found far above sea level on some islands.

Recent discoveries have added to this picture. In the 1990s, USGS hydrologists working in East Maui discovered that numerous perched aquifers in regions of abundant rainfall may be stacked until even the more permeable, intervening layers become saturated with groundwater. The result is a much thicker lens of fresh water, with an upper surface higher than the typical few meters of the conventional model.

At about the same time, researchers at the University of Hawai`i made a startling discovery about the role that soil and ash beds play in guarding the groundwater in the lava flows of Mauna Kea. Their findings resulted from chemical analyses of water in lava flows intercepted by drilling of the hole for the Hawai`i Scientific Drilling Project near Hilo Airport.

As the weight of the island bowed the underlying oceanic crust downward, these low-permeability soil and ash layers, now deep below sea level, prevented fresh and salt water from mixing. Thus, fresh groundwater fed by rainfall in lava-flow aquifers upslope persists well below sea level at the island's edge. The layering of beds in the Hilo drill hole is like a sandwich of alternating fresh- and salt-water-bearing lava flows, each separated by low-permeability layers. Fresh water in the deep layers seeps unseen into the ocean at depths of 300 m (1,000 ft) or more.

Discoveries don't end with Maui and Hawai`i, however. USGS hydrologists working in the Lihu`e basin of Kaua`i have found that low-permeability layers impounded a groundwater system that stands at least 100 m (300 ft) above sea level and more than that below sea level. The low-permeability layers were formed during late-stage volcanism, long after most of the island had been built. On Kaua`i, this volcanic stage, known as the rejuvenated stage, has produced rocks with characteristics that differ from those of more typical shield-building lava-flow aquifers. Natural seepage from the thick freshwater lens plays an important part in maintaining the flow of perennial streams in the basin.

The same low permeability that results in the formation of thicker freshwater lenses on Kaua`i and Maui and causes deeper fresh water to leak offshore from Hilo, however, also limits the rate at which groundwater can be extracted. Overpumping allows underlying brackish water to intrude and diminishes the flow to springs and streams. As with all natural resources, we need to manage our use of groundwater in a sustainable way. Recent and ongoing studies of Hawai`i's geology and hydrology will enable us to do that more effectively.

Kīlauea Activity Update

Surface flows continued to be active on the pali in Royal Gardens subdivision. At the coast, the Waikupanaha ocean entry remains active, but the Kupapa`u ocean entry is no longer active. A deflation-inflation cycle (or DI event) started at the summit on Thursday, July 30 (the date of this writing). DI events often disrupt the lava supply to the east rift zone for several days.

The vent at Kīlauea's summit was dark and quiet all week, producing only a very small quantity of rock dust from small collapses of the vent walls. Volcanic gas emissions have increased over the past two weeks and are currently similar to levels prior to June 30, resulting in high concentrations of sulfur dioxide downwind.

No earthquakes beneath Hawai`i Island were reported felt this past week.