Water is indispensable for human life. For this simple reason, from the beginning of civilization, human beings located their settlements near waters. The scale of the settlements became larger and larger with population increase and eventually grew to modern cities as a result of industrialization in recent two centuries. For any city, natural and artificial waterbodies play multiple roles, such as water resources for water supply, aquatic buffers for local climate regulation, landscape areas for human enjoyment, and aquatic ecosystem to provide ecological services. On the other hand, waterbodies are receiving pollutants from various sources, especially those from human activities. The water after domestic and industrial use is ultimately discharged to receiving waterbodies with considerable pollutant loading. The pollutants from dispersed sources may accumulate on land surfaces in dry days and carried by surface runoff to waterbodies in rainy days. Under serious pollution, waterbodies may no longer be capable to perform their functions well. On the other hand, as water is always moving or being moved within a hydrologic cycle, all issues related to water environmental quality, aquatic ecological function, and the safe use should be put into an integrated framework as indicated here as “sustainable aquatic eco-environmental safety control”.
Water Pollution Control
Water pollution control is a conventional topic but a very important fundamental step. A practical definition of water pollution is: “Water pollution is the addition of substances or energy forms that directly or indirectly alter the nature of the water body in such a manner that negatively affects its legitimate uses”. Water is typically referred to as polluted when it is impaired by anthropogenic contaminants.
A number of water quality indicators are widely used for evaluating the overall water quality, such as COD and/or BOD which indicate the organic pollution level, total nitrogen and phosphorus which indicate the eutrophication level, and fecal coliform or total coliform which indicate the fecal pollution level. To keep the concentrations of these conventional pollutants within the allowable levels it still a heavy task in some countries and regions, especially for waters in densely populated urban area. In most cases, heavy water pollution is mainly caused by the so-called point sources, such as the treated and/or untreated wastewater discharges. In addition to conventional pollutants which are relatively removable by conventional processes such as the secondary treatment in most domestic wastewater treatment plants, attention is also paid to non-biodegradable organic compounds (chemicals from insecticides and herbicides, etc.) and persistent organic pollutants that are resistant to environmental degradation through chemical, biological, and photolytic processes. The concentrations of these pollutants in water are usually low but the toxic effects are considerably high. Their effective removal may need the adoption of sophisticated technologies.
Groundwater Pollution Control consists tasks of two aspects: pollution prevention and remediation of polluted groundwater. Groundwater pollution occurs when pollutants are released to the ground and make their way into groundwater. It can also occur naturally due to the presence of a minor and unwanted constituent, contaminant, or impurity in the groundwater, in which case it is more likely referred to as contamination rather than pollution. Groundwater pollution can occur from on-site sanitation systems, landfill leachate, effluent from wastewater treatment plants, leaking sewers, petrol filling stations, hydraulic fracturing or from over application of fertilizers in agriculture. As in most cases the pollutants are from surfaces, groundwater pollution prevention usually requires the development of land-use zoning maps which include an aquifer vulnerability map referring to the intrinsic (or natural) vulnerability of a groundwater system to pollution, and a source protection map referring to the capture areas around an individual groundwater source, such as a water well or a spring, to especially protect them from pollution. When a groundwater aquifer is polluted or contaminated, it is much more difficult to abate than surface pollution because groundwater can move great distances through unseen aquifers. Pollutants and contaminants can be removed from groundwater by applying various techniques including biological, chemical, and physical treatment technologies. However, if treatment or remediation of the polluted groundwater is deemed to be too difficult or expensive, then abandoning the use of this aquifer’s groundwater and finding an alternative source of water is the only other option.
Aquatic Environmental and Ecological Quality Improvement
Aquatic environmental and ecological quality improvement aims at upgrading the overall quality of waterbodies so as to improve their capability to provide better environmental and ecosystem services. By definition, Environment stresses the surrounding conditions that influence an organism (human or others), while Ecosystem stresses the interactions between organisms and the environment. Therefore, using the term of Aquatic environmental and ecological quality we stress not only the quality of water to influence human life but also its function related to ecosystem services, which corresponds to a target of higher-level water safety.
Regarding source water for drinking water supply, there is usually water quality criteria which represent specific levels of chemicals or conditions that are not expected to cause adverse effects to human health, while regarding environmental water, the highest requirement usually follows the aquatic life criteria representing the specific levels of pollutants in water that are not expected to pose a significant risk to the majority of species. Many important waterbodies may have to meet the quality requirements both for human health and ecological health. This needs a consideration of measures not only to follow current criteria or standards on water quality, but also the requirement for overall safety insurance of water ecological environment.
Nonpoint Source Reduction is considered to be the second step of water pollution control to further improve aquatic environmental and ecological quality. By definition, nonpoint source pollution refers to diffuse contamination or pollution of water that does not originate from a single discrete source. This type of pollution is often the cumulative effect of small amounts of contaminants gathered from a large area, such as a watershed or the whole catchment area of a waterbody. In contrast to pollutants from a point source which may enter the receiving water continuously, the pollutants from nonpoint sources may mainly be carried to a waterbody by surface runoff in rainy days. Many studies indicate that initial stormwater runoff often carries a concentration of pollutants much higher than untreated sewer flow and results in serious water pollution. On the other hand, many highly toxic pollutants from nonpoint sources are those that may not exist in domestic and industrial wastewater, such as pesticides, herbicides, and other agriculture and plantation related chemicals. To certain extent, nonpoint source reduction is more important for securing water safety than further enhancement of point source reduction. To control nonpoint source pollution, many different approaches can be undertaken in both urban and suburban areas. Buffer strips provide a barrier of grass in between impervious paving material like parking lots and roads, and the closest body of water. This allows the soil to absorb any pollution before it enters the local aquatic system. Retention ponds can be built in drainage areas to create an aquatic buffer between runoff pollution and the aquatic environment. Runoff and storm water drain into the retention pond allowing for the contaminants to settle out and become trapped in the pond. The use of porous pavement allows for rain and storm water to drain into the ground beneath the pavement, reducing the amount of runoff that drains directly into the waterbody. Restoration methods such as constructing wetlands are also used to slow runoff as well as absorb contamination.
Aquatic Ecosystem Remediation is to restore the aquatic environmental and ecological condition when a waterbody has been previously polluted. It is extremely important the so-called Remediation should be an action after the goal of pollution control, either regarding point source or nonpoint source, is achieved. Many studies indicated that the deterioration of an aquatic ecosystem is caused by both exogenous and endogenous pollution. Therefore, aquatic ecosystem remediation requires a thorough control of pollutants or contaminants from both exogenous and endogenous sources. In a waterbody, such as a lake or reservoir, bottom sediments are usually the major source of endogenous pollutant source. The strategy for preventing continued release of pollutants from the bottom sediments may include their mechanic or physical elimination (such as sediment dredging), and pollution isolation by chemical, physiochemical, and biological measures. We stress aquatic ecosystem remediation or restoration rather than water quality remediation or restoration because a healthy aquatic ecosystem symbolizes a favorable water quality. There are a number of ecological indicators of healthy water environmental condition, such as freshwater biological indicators including various measures of macroinvertebrate or fish diversity, benthic algal growth and benthic oxygen demand, and habitat indicators including riparian vegetations which provides information on the interface between the land and body of water. Various tools of biotoxicity measurements also assist the evaluation or assessment of the aquatic ecological health condition. These are based on bioassays using selected species of bacteria, algae, daphnia, protozoa, and fishes. Many specific toxicity tests targeting the genotoxicity, estrogenicity, biological community structure are also available for characterizing aquatic eco-environmental quality.
Water Related Urban Safety
By definition, urban safety depends on an environment which ensures safe life of the population on the basis of a combination of factors. All factors forming the local urban safety are divided into groups: natural, architectural, social, environmental, technogenic, and infrastructural. Speaking about the factors related to nature, environment, and even infrastructure, we may list many closely relating to water. Cities are within or connected with water basins where source water can be provided for various purposes of water supply, and used water and storm water can be accommodated. Water supply and drainage facilities are indispensable infrastructures in urban area.
In recent years, the concept of water cycle management is introduced to assist the discussion of water related urban safety. This concept is based on the fact that water utilization if not merely to use water itself but to use the natural hydrological cycle consisting of a series of processes of natural water circulation, natural purification, phase transition and so on. Under natural conditions, the hydrological cycle is always in a dynamic equilibrium state. Human utilization of water results in an addition of small artificial water cycles to the hydrological cycle. This can be explained as a disturbance on the original natural water cycle, but as long as we know how to reduce the extent of human disturbance to the minimum by following the natural manner as far as possible, we can sustain the whole water cycle at a healthy state so that water related urban safety can be insured.
Urban Flooding Control is always the first task for urban safety control. In fact, surface flooding is a natural phenomenon during a heavy storm when rainwater cannot be completely absorbed by soils and penetrate downward, and eventually accumulates on the ground forming surface runoff. The water can then find natural routes to flow into streams and rivers to complete the runoff process. However, as a result of urbanization, the underlying surface has been much altered by construction of buildings, pavement of roads and squares and so on. With less natural soil surface to absorb rainwater, surface runoff forms more rapidly with much high flowrate. This is the main reason for the frequent occurrence of urban flooding in many cities especially the fast urbanized metropolis. There are basically two strategies for urban flooding control. One is the provision and/or expansion of man-made drainage systems with sufficient capacity to accommodate and smoothly discharge to peak flooding flow corresponding to the target of flooding control. Another strategy is to reduce impervious surfaces in streets, parking lots and buildings through natural drainage channels, porous paving, and wetlands, which are collectively called green infrastructure or sustainable urban drainage systems. Areas identified as flood-prone can be converted into parks and playgrounds that can tolerate occasional flooding. Ordinances can be adopted to require developers to retain stormwater on site and require buildings to be elevated, protected by floodwalls and levees, or designed to withstand temporary inundation. Property owners can also invest in solutions themselves, such as re-landscaping their property to take the flow of water away from their building and installing rain barrels, sump pumps, and check valves.
Building Water-Wise Cities is recognized as a comprehensive systematic solution to ensure water related urban safety. The call for building water-wise cities implies a paradigm shift from the highly engineering-dependent conventional urban water system to a brand-new urban water system characterized by engineering in nature. There have been various approaches and conceptual developments toward this direction. In North America, Low Impact Development (LID) is used to describe a land planning and engineering design approach to manage stormwater runoff as part of green infrastructure. This approach implements engineered small-scale hydrologic controls to replicate the pre-development hydrologic regime of watersheds through infiltrating, filtering, storing, evaporating, and detaining runoff close to its source. In Europe, Sustainable Drainage Systems (SuDS) are a collection of water management practices that aim to align modern drainage systems with natural water processes. SuDS efforts make urban drainage systems more compatible with components of the natural water cycle such as storm surge overflows, soil percolation, and bio-filtration. In Australia, Water Sensitive Urban Design (WSUD) is a land planning and engineering design approach which integrates the urban water cycle, including stormwater, groundwater, and wastewater management and water supply, into urban design to minimize environmental degradation and improve aesthetic and recreational appeal. Recently in China, Sponge City became a new urban construction model for flood management, strengthening ecological infrastructure and drainage systems. It can alleviate urban flooding, water resources shortage, and the urban heat island effect and improve the ecological environment and biodiversity by absorbing and capturing rain water and utilizing it to reduce floods. Rain water harvested can be repurposed for irrigation and for home use. It is a form of a sustainable drainage system on an urban scale and beyond.