The erosion in the banks of Mandovi, Terekhol and Chapora rivers and change in their topography has become a regular affair with sand extraction reaching its peak in the recent months, they said, adding, the boom in the real estate sector has fuelled the activity.
Renowned environmentalist and the `Save Mandovi` activist Rajendra Kerkar said the river has been exploited extensively at several places.
"Mandovi is already facing the brunt of incessant ore transportation through ships. If it is abused further for sand extraction, then it will affect the river severely," he said.
A scientist with National Institute of Oceanography said sand extraction at the mouth of a river can let saline water from the ocean rush into its basin.
"Also sidewalls of the river bank collapse if extraction goes deeper in the basin. The sand extraction activity has a huge environmental impact," he added.
Experts have voiced concern that sand mining operations may invoke profound ecological changes affecting the entire ecosystem.
Mining is done up to a depth of six metres and within 10 metres from the high tide line, Mines department officials said, adding, sand extraction is not allowed within 25 metres of the river bank.
As per the state Mines and Geology department records, Goa has issued 59 licences for sand extraction - 5 in Chapora, Mandovi 26 and Terekhol 28.
Mines department officials concede that they do not have enough man-power and machinery at their disposal to arrest irregularities and check flouting of extraction norms.
"There is no separate section which looks after sand extraction. There are several issues at our disposal. Illegal sand extraction is not the only worry," a senior official said.
The department official said till a few years, sand was extracted only from Terekhol river but recently it was also permitted in Mandovi and Chapora rivers.
Earlier, Colvale, a village 40 kms away from Panaji, was known for extraction. But now things have changed.
State Tourism minister Nilkant Halarnkar, who formerly owned a licence to extract sand, says that there is hardly any sand left for mining in Colvale now.
"There is no sand left in the river," he said, adding, the amount of sand that is being extracted at Colvale is just 25 per cent of what it was in past.
"Though I left this trade eight years ago, several new contractors have jumped the bandwagon," he added.
The boom in sand extraction in Goa started after the Karnataka government disallowed sand from neighbouring Karwar to be brought into the state.
"Around 500 trucks from Goa and 1,000 from Karnatakaare left without work due to the ban. Also there is other machinery like cranes which are lying idle," said South Goa Sand Transportation Association treasurer Moti Desai.