The Arenal reservoir in Costa Rica

The Arenal reservoir with the active Arenal volcano in the background.

This reservoir is situated close to the water divide. During natural conditions this area was drained by Rio Arenal towards the Atlantic. Through the construction of the Sangregado dam in 1979 the water level of the former Laguna Arenal was raised from about 515 m, forming the Arenal reservoir with an upper storage level of 546 m a.s.l. The water flow was reversed, and the water from the reservoir and its drainage area is now draining towards the Pacific.

Stations for coring (1 - 38) and for grab sampling (I - XII) in the Arenal reservoir. Grid net = 1 km. Highest contour line = 550 m a.s.l. Equidistance: 10 m.

Sediment cores were sampled at selected locations in the Arenal reservoir and X-rayed in order to document the vertical sequence of sedimentary structures and to determine the amount of sediment deposited on top of the pre-reservoir surface. Most of the cores (30 of 38) were sampled in March 1995.

Partly overlapping radiographs of the upper and the lower part of sediment core F1 from the Arenal reservoir. Black, round and elliptical spots in the radiographs, that mark the presence of gas in bubble-phase, characterize especially the organic layer on top of the sandy pre-reservoir deposits in the bottom of the core.

As exemplified by core F1 the level of the pre-reservoir surface was visible (also to the naked eye) in most of the sampled sediment cores, which simplified the determination of average sedimentation rates. Also at several other stations the pre-reservoir deposits were sandy in the uppermost part and capped by gas-rich organic layers, indicating the presence of former floodplain and wetland deposits. This is also reflected in the downcore variation in content of solids, as shown by the curve below.

Downcore variation in content of solids in sediment core A1 from coring station 8.

Radiographic comparison between the upper part of cores E1 and D9 from the Arenal reservoir, sampled at a distance of about 2 km from each other (stations 28 and 34 respectively). The given chronology is somewhat uncertain.

A rhytmic variation between harder (coarser and more minerogenic) and softer (finer grained and more organic) layers, deposited during the rainy and the dry seasons respectively, are visible in several cores from this reservoir. The radiographs of cores E1 and D9 illustrate, that in some areas the spatial distribution of the sedimentary layers could be followed by core-to-core correlations.

Sedimentation-compression curve down to a depth of 38.5 cm in sediment core E7 from station 18 in the Arenal reservoir.  E.o.p. = effective overburden pressure.

The modern deposits in the reservoir mainly consist of medium silt and they are very little compacted, as examplified by the sedimentation-compression curve of core E7. The average dry bulk density of the sediment deposited during the operational period of the Arenal reservoir is in general very low and was in 1995 in large areas lower than 0.3. The thickness of these layers, especially the softer parts, may therefore decrease considerably with increasing depth of burial because of gravitational compaction.

The results of the X-ray radiographic studies indicate that the useful life of the Arenal reservoir is very long. The sedimentation rate is higher in the eastern than in the western part of the reservoir. Especially in the former lake and wetland areas, the former Laguna Arenal, the rate of sedimentation was very low during the operational period 1979 - 1995. The mean annual sedimentation rate here amounted to only a few mm, corresponding to less than half a kg/m². The rate of sedimentation was highest in front of Río Chiguito, where a delta was formed, that with time will reach the opposite shore and thus divide the Arenal reservoir into two basins.

The mean total, annual amount of solids deposited below 530 m a.s.l., that is mainly within the dead storage area, amounted probably to about 150,000 tonnes during the above given operational period. About 1/6 of this amount was deposited in the reservoir to the west of the delta front area of Río Chiquito, about 2/6 within the delta front area, assumed to cover about 2 km², and about 3/6 to the east of the delta front area of Río Chiquito.

During the operational period 1979 - 1995 the calculated mean annual increase in sediment volume (annul sediment accumulation minus annual sediment compaction) below 530 m a.s.l. was of the order of 500,000 m³, corresponding to a mean annual increase in thickness of the modern reservoir deposits below this level of about 7 mm. These figures do not include delta deposits upstream the 530 m contour level and they do not take into consideration the increase in volume due to upfloating "gamalotes" (peat that was inundated by the reservoir).

The above given values of sedimentation rates are uncertain but no doubt of the right order. The dry bulk density of the sediment deposited during the given operational period of the reservoir was in general very low. The thickness, especially of the softer layers, will therefore decrease considerably with increasing depth of burial because of gravitational compaction. The reservoir deposits below the lower storage level will probably decrease to about half its volume in 1995 (an increase in dry bulk density from 0.3 to about 0.6) during a period of 100 years, provided that the rate of sediment accumulation will be the same as during the above given operational period.

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