Optional theme: Freshwater - drainage basins practice questions.

In a steep upland reach of the River Tees in northern England, turbulent flow forces water and compressed air into joints in the resistant dolerite, and the repeated pressure prises whole blocks out of the bed. Which erosional process is described?

• Abrasion (corrasion) by the bedload
• Attrition between transported particles
• Hydraulic action
• Solution (corrosion) of the bedrock

Answer: Hydraulic action

Hydraulic action is the mechanical force of water and trapped air alone; abrasion needs sediment as a tool, attrition only wears down the load itself, and dolerite, unlike limestone, is effectively insoluble on the timescale of channel erosion, so solution removes a negligible amount here.

In 1998 a flood on the Ganges-Brahmaputra-Meghna delta in Bangladesh inundated roughly two thirds of the country, and standing water remained for about two months. Which factor best explains the exceptional duration of the flooding?

• The delta is underlain by impermeable granite, so no infiltration occurs
• Himalayan snowmelt reached the delta long before any monsoon rain fell
• Peak flows of three large rivers coincided on an almost flat delta
• Tectonic uplift of the delta blocked the distributary channels during the flood

Answer: Peak flows of three large rivers coincided on an almost flat delta

With a gradient of only centimetres per kilometre, water on the delta drains extremely slowly, so simultaneous peaks from the Ganges, Brahmaputra and Meghna pond instead of passing through. Duration, not depth alone, is what makes delta floods so damaging.

A local authority is choosing between four flood-management measures for a town on a floodplain. Which of them is non-structural (soft) engineering?

• Raising and reinforcing the earth embankments along both banks
• Straightening and deepening the channel to move water away faster
• Building an underground flood-relief tunnel beneath the town centre
• Banning new housing in the highest-risk floodplain zone

Answer: Banning new housing in the highest-risk floodplain zone

Soft engineering changes what people do rather than what the river does. The other three alter the channel or add built structures, and each of them passes water, and therefore risk, on to settlements downstream.

In August 2014 the city of Toledo, Ohio, told residents not to drink tap water after a cyanobacteria bloom in the western basin of Lake Erie released the toxin microcystin. Which chain of events best explains the bloom?

• Acid rain lowered the lake's pH, letting cyanobacteria outcompete other algae
• Chloride from winter road salting raised water density and trapped nutrients
• Warm cooling water from power stations raised dissolved oxygen and fed the bloom
• Phosphorus in runoff from farmland enriched the warm, shallow water

Answer: Phosphorus in runoff from farmland enriched the warm, shallow water

Blooms need a nutrient supply, mainly phosphorus from fertiliser and manure, plus warm, shallow, slow-moving water, and the western basin of Lake Erie provides both. Treating the symptom at the water works does nothing about the source in the catchment.

Groundwater is withdrawn from the High Plains (Ogallala) Aquifer of the central USA far faster than the semi-arid climate can recharge it. Which term describes this pattern of use?

• Salinisation of the aquifer
• Eutrophication of the aquifer
• Interbasin transfer of groundwater
• Over-abstraction, or water mining

Answer: Over-abstraction, or water mining

When abstraction exceeds recharge the store is being consumed rather than used, so the water table falls, pumping costs rise and shallow wells fail. Much of the Ogallala's water accumulated under wetter Pleistocene conditions and is effectively non-renewable.

From the 1970s millions of shallow tube wells were sunk across Bangladesh to replace bacterially contaminated surface water. Many were later found to deliver water above the World Health Organization guideline value for arsenic. Where does that arsenic come from?

• Industrial effluent discharged into the Ganges upstream
• Arsenical pesticides sprayed on rice paddies since the 1970s
• Arsenic-bearing sediments in the delta's alluvial aquifer
• Seawater intrusion carrying arsenic inland from the Bay of Bengal

Answer: Arsenic-bearing sediments in the delta's alluvial aquifer

The arsenic is geogenic: it is released from Himalayan-derived sediments under the reducing conditions of the shallow aquifer. A switch that solved a microbiological problem therefore created a chemical one, which is why well testing matters more than blaming industry.