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Mudflow may contaminate groundwater in East Java
The quality of groundwater flowing to Surabaya and Gresik in East Java is likely to drop and will be contaminated with heavy metals from the mudflow in Sidoarjo in the next decades, an expert says
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he quality of groundwater flowing to Surabaya and Gresik in East Java is likely to drop and will be contaminated with heavy metals from the mudflow in Sidoarjo in the next decades, an expert says.
Groundwater for the two cities originates from springs located on the slopes 3,339-meter high Mount Arjuna and the 3,156-meter high Mount Welirang, located in Pasuruan and Mojokerto regencies.
Hydrologist Heru Hendrayana of Yogyakarta's Gadjah Mada University said that from the mountains, the groundwater flows to the north through Pasuruan, Surabaya, and Gresik before it empties into the Java Sea.
"Although flowing slowly, the groundwater passes through the cities because it flows to the north," Heru told The Jakarta Post in Pasuruan.
He said that groundwater contamination occurred because water, which moves through the aquifer or layers containing saturated water, meets the aquifer layers in Porong mudflow site in Sidoarjo. The aquifer layers in Porong are polluted with heavy metals due to subsidence.
"Satellite images and research also found many fractures in the soil that can raise the contaminants," said the former expert member of an independent team monitoring the mudflow.
He said that one factor that could affect groundwater quality was geological-based.
"Theoretically, groundwater quality is considered high if water, which contains natural minerals that are good for health, passes through volcanic rocks far away from human activity, is not contaminated and goes through an effective filtering process," said Heru.
The groundwater that flows to Surabaya and Gresik is high quality and has a high quantity because it comes from a volcanic area, whose aquifer layers is relatively free of contaminants; contains naturally balanced minerals such as calcium, sodium, magnesium and potassium, as well as has an abundant amount of moisture.
Groundwater in this area, he said, can be found at two depths - a depth of between 30 and 50 meters, and between 80 and 100 meters.
Currently, most residents in Surabaya and Gresik still use groundwater sources at a depth of between 30-50 meters.
One of the world's largest bottled water factories, Danone Aqua in Pasuruan, make use of the groundwater at depths between 80-100 meters for its raw material supply at a capacity of up to 50 liters per second.
"We guarantee that the quality of artesian water source used by Aqua is not affected by the Porong geological activities since the location of our springs is higher than Porong," said Danone Aqua's water resource manager Wahyu Triraharja.
In the event of a subsidence, the filtration process that takes place naturally, through layers of volcanic rocks since millions of years ago, will also be altered.
Subsidence also affects the porosity - the total volume of air space between particles in mass - and permeability or the ability of rocks to pass groundwater through aquifer layers.
"In principle, the quality of groundwater varies in each place, depending on the types of rock aquifer and the contact time between water and rock minerals, and the types of human activity undertaken," Heru said.
