Read the following passage and answer the items that follow. Your answers to these items should be based on the passage only.
The law in many parts of the world increasingly restricts the discharge of agricultural slurry in to water courses. The simplest and most economically sound practice returns the material to the land as semisolid manure or as sprayed slurry. This dilutes its concentration in the environment to what might have occurred in a more primitive and sustainable type of agriculture and converts pollutant into fertilizer. Soil microorganisms decompose the organic components of sewage and slurry and most of the mineral nutrients become available to be absorbed again by the vegetation. The excess input of the nutrients, both nitrogen and phosphorus – based, from agricultural runoff (and human sewage) has caused many ‘healthy’ oligotrophic lakes (low nutrient concentrations, low plant productivity with abundant water weeds, and clear water) to change to eutrophic condition where high nutrient lead to high phytoplankton productivity (sometimes dominated by bloom-forming toxic species). This makes the water turbid, eliminates large plants and, in the worst situations, leads to anoxia and fish kills; so called cultural eutrophication. Thus important ecosystem services are lost, including the provisioning service of wild-caught fish and cultural service associated with recreation.
The process of cultural eutrophication of lakes has been understood for some time. But only recently did scientists notice huge ‘dead zones’ in the oceans near river outlets, particularly those draining large catchment areas such as Mississippi and North America and the Yangtze in China. The nutrient enriched water flows through streams, rivers and lakes and eventually to the estuary and ocean where the ecological impact may be huge, killing virtually all invertebrates and fish in areas up to 70,000 km2 in extent. More than 150 sea areas worldwide are now regularly starved of oxygen as a result of decomposition of algal blooms, fueled particularly by nitrogen from agricultural runoff of fertilizers and sewage from large cities. Oceanic dead zones are typically associated with industrialized nations and usually lie off countries that subsidize their agriculture, encouraging farmers to increase productivity and use more fertilizer.
According to the passage, why should the discharge agricultural slurry into watercourses be restricted?
The passage provides multiple reasons for restricting the discharge of agricultural slurry into watercourses. All three statements are supported by the text:
Statement 1: Losing nutrients in this way is not good practice economically. The passage explicitly states, "The simplest and most economically sound practice returns the material to the land as semisolid manure or as sprayed slurry." This implies that discharging slurry into water bodies is economically unsound, as it prevents the beneficial reuse of nutrients as fertilizer and represents a loss of valuable resources.
Statement 2: Water courses do not contain microorganisms that can decompose organic components of agricultural slurry. The passage notes, "Soil microorganisms decompose the organic components of sewage and slurry and most of the mineral nutrients become available to be absorbed again by the vegetation." This indicates that the primary environment for effective microbial decomposition and nutrient recycling of slurry is soil, not watercourses, which lack the necessary microbial communities for this process.
Statement 3: The discharge may lead to the eutrophication of water bodies. The passage details how "excess input of the nutrients, both nitrogen and phosphorus – based, from agricultural runoff" (which includes slurry) causes "cultural eutrophication." This process transforms healthy oligotrophic lakes into eutrophic conditions, leading to high phytoplankton productivity, water turbidity, elimination of large plants, anoxia, and fish kills. It also mentions the formation of oceanic "dead zones" due to nutrient-enriched water from such discharges.
Options 1, 2, and 3 are incorrect because they only partially identify the reasons for restricting agricultural slurry discharge. The passage clearly supports all three statements as valid reasons, making option 4 the comprehensive and correct choice.