Archis R. Ambulkar
Since the industrial revolution, societies across the globe have observed significant urbanization and population growth. Newer technologies, industries, and manufacturing plants have evolved over the period to develop sophisticated infrastructures and amenities for mankind. To achieve this, communities have utilized and exploited natural resources, resulting in sustained environmental degradation and pollution. Among various adverse ecological effects, nutrient contamination in water is posing serious problems for the water bodies worldwide.
Nitrogen and phosphorus are the basic constituents for the growth and reproduction of living organisms and occur naturally in the soil, air, and water. However, human activities are affecting their natural cycles and causing excessive dumping into the surface and groundwater systems. Higher concentrations of nitrogen and phosphorus-based nutrients in water resources lead to eutrophication, reduction in sunlight, lower dissolved oxygen levels, changing rates of plant growth, reproduction patterns, and overall deterioration of water quality. Economically, this pollution can impact the fishing industry, recreational businesses, property values, and tourism. Also, using nutrient-polluted lakes or rivers as potable water sources may result in excess nitrates in drinking water, production of disinfection by-products, and associated health effects.
Nutrients contamination in water commonly originates from point and non-point sources. Point sources are the specific discharge locations, like wastewater treatment plants (WWTP), industries, and municipal waste systems; whereas, non-point sources are discrete dischargers, like agricultural lands and storm water runoffs. Compared to non-point sources, point sources are easier to identify, regulate, and treat. WWTPs receive sewage from domestic, business, and industrial settings. With growing pollution concerns, nutrients removal and recovery at treatment plants is gaining significant attention. Newer chemical and biological nutrient removal processes are emerging to treat wastewater. Nitrogen removal mainly involves nitrification-denitrification processes; whereas, phosphorus removal includes biological uptake, chemical precipitation, or filtration. In regards to non-point sources, authorities are encouraging best management practices to control pollution loads to waterways.
Governments are opting for novel strategies like source nutrient reduction schemes, bioremediation processes, stringent effluent limits, and nutrient trading programs. Source nutrient reduction strategies such as discouraging or banning use of phosphorus-rich detergents and selective chemicals, industrial pretreatment programs, and stormwater management programs can be effective by reducing nutrient loads to WWTPs. Bioremediation techniques such as riparian areas, natural and constructed wetlands, and treatment ponds can capture nutrients from agricultural lands or sewage treatment plant effluents. Nutrient trading programs allow purchase/sale of equivalent environmental credits between point and non-point nutrient dischargers to manage overall nutrient discharges in watersheds at lower costs.
Nutrient pollution impacts are quite evident and documented in many parts of the world. Governments and environmental organizations are undertaking several waterways remediation projects to improve water quality and restore aquatic ecosystems. Shrinking freshwater reserves and rising water demands are compelling communities to make efficient use of the available water resources. With smarter choices and useful strategies, nutrient pollution in the water can be contained to a reasonable extent. As responsible members of the community, it is important for us to understand this key environmental issue as well as to learn the current and future needs to alleviate this problem.
Gerrit de Rooij
Henry Darcy was an engineer who built the drinking water supply system of the French city of Dijon in the mid-19th century. In doing so, he developed an interest in the flow of water through sands, and, together with Charles Ritter, he experimented (in a hospital, for unclear reasons) with water flow in a vertical cylinder filled with different sands to determine the laws of flow of water through sand. The results were published in an appendix to Darcy’s report on his work on Dijon’s water supply. Darcy and Ritter installed mercury manometers at the bottom and near the top of the cylinder, and they observed that the water flux density through the sand was proportional to the difference between the mercury levels. After mercury levels are converted to equivalent water levels and recast in differential form, this relationship is known as Darcy’s Law, and until this day it is the cornerstone of the theory of water flow in porous media. The development of groundwater hydrology and soil water hydrology that originated with Darcy’s Law is tracked through seminal contributions over the past 160 years.
Darcy’s Law was quickly adopted for calculating groundwater flow, which blossomed after the introduction of a few very useful simplifying assumptions that permitted a host of analytical solutions to groundwater problems, including flows toward pumped drinking water wells and toward drain tubes. Computers have made possible ever more advanced numerical solutions based on Darcy’s Law, which have allowed tailor-made computations for specific areas. In soil hydrology, Darcy’s Law itself required modification to facilitate its application for different soil water contents. The understanding of the relationship between the potential energy of soil water and the soil water content emerged early in the 20th century. The mathematical formalization of the consequences for the flow rate and storage change of soil water was established in the 1930s, but only after the 1970s did computers become powerful enough to tackle unsaturated flows head-on. In combination with crop growth models, this allowed Darcy-based models to aid in the setup of irrigation practices and to optimize drainage designs. In the past decades, spatial variation of the hydraulic properties of aquifers and soils has been shown to affect the transfer of solutes from soils to groundwater and from groundwater to surface water. More recently, regional and continental-scale hydrology have been required to quantify the role of the terrestrial hydrological cycle in relation to climate change. Both developments may pose new areas of application, or show the limits of applicability, of a law derived from a few experiments on a cylinder filled with sand in the 1850s.