Murmansk Marine Biological Institute (KSC, RAS), Russia, Russian Federal Research Institute of Fishery and Oceanography, Russia

Offshore oil and gas fields development operations (drilling, laying subsea pipelines, dumping, and other hydro-engineering operations) cause a significant increase in natural background of suspended material. At the same time, in zones of high turbidity, suspended material is the factor that is able to cause depression of the main life processes in sea organisms and even to cause them death. In such situations there is a necessity to assess respective damage and calculate the sum of compensatory payments for restoration of marine biological resources. For this purpose, a uniform procedure is authorized in the Russian Federation (Temporary methodology…, 1989). The practice of such calculations shows that damage rates are usually overrated because of incorrect account of some parameters, the impact period of suspended material in particular.

The purpose of the present work is to argue the necessity of taking into account the impact period of mineral suspension on marine living organisms, and to develop recommendations on how to consider this factor when calculating damage to marine biological resources.

At all kinds of hydro-engineering operations on shelf, the time of existence and scales of distribution of a turbid cloud of suspended material are defined by a number of factors among which the following are of particular significance: means of dredging (use of various mechanisms – drags, suction dredgers, giants etc.); means of transportation of dredged material to the dumping area; means of dumping; the volume of dredged material dumped; depths at the site of initial spread of the turbid cloud; the grain-size of suspended materials; velocity of the sea current.

Time and spatial scales of turbid clouds of suspended material and their characteristics can vary in a wide range. Spatial and temporal range of suspension clouds can be determined with a high degree of approximation since each case demands special calculations.

Further on the most typical concentrations of suspended material and time of its existence are discussed for the main hydro-engineering operations.
Offshore drilling. Modeling of drilling agent and slime dumping during well drilling (Sakhalin shelf, depth less than 50 meters) have shown the following (Research report, 2001): at volley discharge reduction of the maximum concentration to 5 mg/l occurs in six hours after dumping, during high-intensity discharge it takes less than three hours, and after five hours the concentration is less than 1 mg/l. The typical scale of pollution is hundreds of meters.

Laying an underwater pipeline. According to modeling results (Klevany, Shavykin, 2008) turbid clouds with a concentration of more than 10 mg/l exist for 5-6 days, with a concentration of more than 100 mg/l – 18-40 hours. The time of existence of a turbid cloud with a concentration of more than 1000 mg/l is 14-16 hours. On the basis of results of field observations some authors (Wilber, Clarke, 2001; Nightingale, Simenstad, 2001) have noticed that the time of existence of a suspension cloud with a concentration of 10 mg/l and above usually does not exceed 4 days.

Dumping. During dredged sediment disposal operations concentrations of suspended material rapidly decrease with distance from the source (Hayes, 1986; Collins, 1995; Herbich, Brahme, 1991). As a rule, concentration of suspension in a turbid cloud quickly decreases and in 1-1.5 hours becomes less than 10 mg/l (Borisov, Goncharov, 1988).

Dredging. During dredging operations high concentrations of suspended material exist for a short period of time: 1 000 mg/l and above within a day and 100 mg/l and above within 1-3 days.

Based on experimental data (Research report, 2000) used in determination of the Maximum Permissible Concentration (MPC) of suspended material for shelf seas with Artemia salina as a test object recommended by ISO 10253:1995 for assessments of water quality, the following mortality curves for zooplanktonic organisms have been obtained.

Three stages can be identified on each diagram: the first stage – no mortality is registered (2 to 5 days), the second stage is characterized by a rapid growth of mortality rate, and the third stage is characterized by a slow growth of mortality for all the concentrations described.
Concentrations of 10 mg/l (MPC for shelf seas) and less are not considered because such concentrations are permissible and do not cause mortality of organisms. Therefore they should not be taken into account at damage calculations.

It is necessary to notice that 50 % mortality is observed only when concentrations reach 1 000 mg/l and more with impact duration of 21 days. Comparing mortality curves with average values of suspension cloud duration it is possible to notice that 50 % mortality of zooplankton during various hydro-engineering operations can hardly occur. In most cases time of existence of suspension clouds is consistent with the period of null mortality (2 to 5 days for various concentrations).

In most cases suspension clouds with high concentration (> 10 mg/l) exist no more than 2-6 days, the death rate for this period is less than 30 %. Thus the value of 30 % can be recommended as a maximum value of zooplankton mortality in calculations of damage.

However, at present the Federal Service for Veterinary and Phytosanitary Surveillance (Rosselkhoznadzor) recommends the following concentration and mortality rates to be used in calculations: 20-100 mg/l causing 50 % mortality of zooplankton and 100 mg/l and above causing 100 % mortality. There are a number of other organizations that use similar values. At the same time in all these calculations the time of existence of turbid clouds with increased concentrations is not considered. Such approach strongly overestimates counted damage.

Thus, at high concentrations of suspended material in water (100 mg/l and above), death of organisms occurs by stages with the initial stage of about 2 days for a concentration of 1000 mg/l and up to 5 days for 100 mg/l characterized by absence of mortality.

Generally, zooplankton mortality rate from effects of high concentrations of suspended material (1000 mg/l and less) during hydro-engineering operations does not exceed 30 %.



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