How have the environmental changes that have occurred in recent decades in French Guiana, including gold extraction in the hinterland, affected the Maroni River? A team involving IRD researchers analysed changes in sediment flows in this water course since 2000. Their findings are alarming.
The concentration of suspended soil particles, or sediments, in the Maroni River has dramatically increased over the past decade. This is due to the increase in gold mining activity in the region drained by the river and its tributaries. This is revealed by a study(1) conducted by Marjorie Gallay, former doctoral student at Cayenne’s IRD Centre, and Jean-Michel Martinez, research director at the Toulouse Geosciences environment laboratory, who supervised the young researcher’s thesis. “Our results call for emergency measures to preserve the Maroni River”, warns Marjorie Gallay.
A natural border between Suriname and French Guiana, the 612-kilometre-long Maroni River flows through one of the world’s largest tropical forests. This water course, crucial for the exceptional biodiversity of this region, also supplies and transports local populations. Problem: it is downstream of the gold mines which have developed exponentially inland since the 1990s. Gold extraction aggravates deforestation and the destruction of watershed soil and riverbed, while intensifying erosion mechanisms. As a result, the influx of sediment into the river through rainwater increases.
The researchers analysed hundreds of sediment concentration measurements taken in the Maroni River from 2001 to 2015. Some were taken by IRD’s HyBAm observatory(2)– who “have measured the physical-chemical parameters of the Maroni River every month since 2004”, states Jean-Michel Martinez; others by Marjorie Gallay, “from a canoe, at 81 points on the Maroni River”; others by MODIS sensors(3) remote sensing instruments on board NASA’s Terra and Aqua satellites.
It was ultimately found that, from 2001 to 2015, the suspended sediment concentration in the Maroni River increased by… 239%, from 10 mg/L on average in 2001 to nearly 36 mg/L in 2015! Jean-Michel Martinez is still astonished: “We worked on a number of large rivers such as the Mekong River in Asia or the Niger River in Africa, but we had never seen such a significant change…”.
… which could create several problems
The researchers subsequently examined several factors likely to explain their findings: rainfall variations in the region, land use changes linked to agriculture and urbanisation, and extensive mining-related deforestation. In fact, only the latter varied significantly: while in 2000 its cumulative surface area was only 4,821 hectares, it increased fivefold in 2016 to 24,463 ha. This leads to the conclusion that the increase in suspended sediment in the Maroni River is primarily due to the mining activity.
Excess sediment in the Maroni River diminishes water transparency and may therefore affect the survival of the river’s species. This may also limit the navigability of the Maroni River. Last but not least, sediment carries trace elements, including mercury, which are likely to pollute the river and contaminate the populations along its shores. This is why it is crucial that we continue to assess changes in the sediment content of the Maroni River and their possible consequences. This task will take a few more years to complete.
1. Marjorie Gallay, Jean-Michel Martinez, Sebastien Allo, Abrahan Mora, Gérard Cochonneau, Antoine Gardel, Jean-Claude Doudou, Max Sarrazin, Franck Chow-Toun, Alain Laraque, Land Degradation & Development , 6 septembre 2018 ; Impact of land degradation from mining activities on the sediment fluxes in two large rivers of French Guiana
2. Pour « HYdrologie et Biogéochimie du Bassin Amazonien »
3. Pour« ModerateResolution Imaging Spectroradiometer »
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