Open Limestone Channel Research Articles

Geomorphic-based mine rehabilitation coupled with AMD chemical stabilisation in sulphide-rich ore deposits and soils: insights from a pioneering intervention at the Lousal mine, Iberian pyrite Belt

ABSTRACT The Lousal mine rehabilitation project proactively planned strategic milestones around key local geomorphic landforms and geochemical characteristics. GeoFluv™ method was used to design a rehabilitation topography mimicking natural landforms, on which a built-up soil cover with chemical buffering capabilities and open limestone channels were implemented across the intervention areas. Once these landform restoration works were completed, positive effects are observed in March 2022, with the native vegetation cover (the third main component of the restoration plan) established in most of the recovered area and a visible water quality improvement to the AMD treatment system water. Potentially Toxic Elements (PTEs) concentrations have reduced significantly after the reclamation actions, especially Fe (404 to 34 mg L−1), Zn (65.7 to 15.8 mg L−1) and Cd (122 to 0.8 µg L−1) concentrations. Minor problems detected have been processes of rill erosion associated with the inflow of adjacent watersheds, and the creation of ephemeral reducing conditions resulting from the leaching of the organic amendment of the topsoil. The Lousal mine reclamation project is the first built example in Europe where the concept of geomorphic reclamation has been combined with geochemical remediation in an area with intense acid mine drainage formation. Detailed monitoring of this project’s restorative progression throughout 2024 and beyond, should offer learning opportunities and innovations which will benefit future rehabilitation projects, with comparable underlying features.

Remediation technologies for acid mine drainage: Recent trends and future perspectives

Acid mine drainage (AMD) is a highly acidic solution rich in heavy metals and produced by mining activities. It can severely inhibit the growth of plants, and microbial communities and disturb the surrounding ecosystem. In recent years, the use of different bioremediation technologies to treat AMD pollution has received widespread attention due to its environment-friendly and low-cost nature. Various active and passive remediation technologies have been developed for the treatment of AMD. The active treatment involves the use of different chemical compounds while passive treatments utilize natural and biological processes like constructed wetlands, anaerobic sulfate-reducing bioreactors, anoxic limestone drains, vertical flow wetlands, limestone leach beds, open limestone channels, and various organic materials. Moreover, different nanomaterials have also been successfully employed in AMD treatment. There are also reports on certain plant growth-promoting rhizobacteria (PGPR) which have the potential to enhance the growth and productivity of plants under AMD-contaminated soil conditions. PGPR applied to plants with phytoremediation potential called PGPR-assisted phytoremediation has emerged as an economical and environment-friendly approach. Nevertheless, various approaches have been tested and employed, all the approaches have certain limitations in terms of efficiency, secondary pollution of chemicals used for the remediation of AMD, and disposal of materials used as sorbents or as phytoextractants as in the case of PGPR-assisted phytoremediation. In the future, more research work is needed to enhance the efficiency of various approaches employed with special attention to alleviating secondary pollutants production and safe disposal of materials used or biomass produced during PGPR-assisted phytoremediation.

Neutralization of Acid Mine Drainage Contaminated Water and Ecorestoration of Stream in a Coal Mining Area of East Jaintia Hills, Meghalaya

The unscientific mining of coal without environmental safeguards in Jaintia Hills, Meghalaya has adversely affected the water resources of the area, leading to streams with a pH of 3–5. The acidic stream water is not suitable for human use and lacks aquatic life. The prevailing situation demanded immediate neutralization and eco-restoration. Considering the need, the water of Moolawar stream in Mukhaialong village, East Jaintia Hills District, Meghalaya, India was neutralized by constructing an open limestone channel (OLC) using locally available limestone. The OLC raised the pH of the stream water from 4.31 to 6.57. The near-neutral pH has promoted the reappearance of many aquatic flora and fauna, including two species of fishes and some insects. The OLC was found to be cost effective and technically feasible in this rural area and thus prompted the construction of more such projects for improvement of water quality and ecorestoration of degraded streams. Details of the study and associated improvements in water quality and aquatic ecology are reported.

Tailing’s geomorphology of the San Quintín mining site (Spain): landform catalogue, aeolian erosion and environmental implications

The research on tailings deposits regarding geomorphic approaches, aeolian activity and integrated physical–chemical dynamics and instability is almost inexistent. This work performs such analysis at the San Quintin Mine site, located on the Alcudia Valley region of central-south Spain. The first evidence of mining activity on San Quintin goes back to 1559 and the last one to 1988. The mine activity here produced two very different deposits. ‘Old tailings’ correspond to early concentration procedures between 1889 and 1923. The resulting piles look chaotic and the tailings vertically and laterally alternate with rock wastes. These deposits have been eroded, mostly, by fluvial processes. ‘Modern tailings’ deposits correspond to more efficient froth flotation treatments, between 1973 and 1988, when part of the old tailings were reprocessed. The grain size of these latter tailings is more homogenous, mostly sandy, which favour intense aeolian erosion. By means of a detailed inventory, we were able to identify and catalogue both fluvial and aeolian landforms, along with those produced by weathering or mass movement. We also quantified aeolian erosion, obtaining a single net wind transport rate of 12.6 t ha−1 year−1, related to WSW winds. From an understanding of the geomorphic activity of the mined area, we propose guidelines for the restoration and remediation of the site, comprising wind erosion prevention measures for the modern tailings and physical stabilization through geomorphic restoration for the old tailings, combined with chemical remediation measures that have been proved useful on similar cases: open limestone channels connected to constructed wetlands.

Performance of an open limestone channel for treating a stream affected by acid rock drainage (León, Spain).

The generation of acid rock drainage (ARD) was observed after the oxidation dissolution of pyrite-rich black shales, which were excavated during the construction of a highway in León (Spain). ARDs are characterized by the presence of high concentrations of sulfate and metals (Al, Fe, Mn, Zn, Cu, Co, Ni, Th, and U) that affect the La Silva stream. Dissolved element concentrations showed values between one and four orders of magnitude higher than those of natural waters of this area. A passive treatment system was constructed; the aim of which was to improve the quality of the water of the stream. This work provides a hydrochemical characterization of the La Silva stream after its transit through the different elements that constitute the passive treatment system (open limestone channel (OLC), small ponds, and a wetland), during its first year of operation. The passive treatment system has two sections separated by a tunnel 230m long. The first section, which stretches between the highway and the tunnel entrance, is an OLC 350m long with a slope of 16%. The second section, which stretches from the tunnel exit to the end wetland, has a length of 700m and a slope of 6%; it is in this section where six small ponds are located. In the first section of this passive treatment system, the OLC was effectively increasing the pH from 3 to 4-4.5 and eliminating all of the dissolved Fe and the partially dissolved Al. These elements, after hydrolysis at a pH 3-3.5 and 4-4.5, respectively, had precipitated as schwertmannite and hydrobasaluminite, while other dissolved metals were removed totally or partially for adsorption by the precipitates and/or by coprecipitation. The second section receives different inputs of water such as ARDs and natural waters. After exiting the treatment system, the stream is buffered by Al at a pH of 4-4.3, showing high Al concentrations (19-101mg/L) but with a complete removal of dissolved Fe. Unfortunately, the outflow shows similar or higher acidity than the inflow into the system due to the discharge of ARDs (mainly from the tunnel) that is received and to the existence of a natural stream, which is affected by a waste-rock pile. The predictions and calculations necessary for the design of any remediation/attenuation techniques are quite difficult. Despite the fact that the selected design is the most adequate one for this valley and type of passive treatment system (including adequate slopes), we must admit that the physicochemical characteristics of the ARD were not the most appropriate according to the literature. Moreover, during the design, engineers were unaware of the existence of the inflow from two highly polluting sources, which have rendered the passive treatment system ineffective and which therefore suggest that certain improvement measures could be considered.

Remediation of Acid Mine Drainage-Impacted Water

The formation of acid mine drainage (AMD), a highly acidic and metal-rich solution, is the biggest environmental concern associated with coal and mineral mining. Once produced, AMD can severely impact the surrounding ecosystem due to its acidity, metal toxicity, sedimentation and other deleterious properties. Hence, implementations of effective post-mining management practices are necessary to control AMD pollution. Due to the existence of a number of federal and state regulations, it is necessary for private and government agencies to come up with various AMD treatment and/or control technologies. This review describes some of the widely used AMD remediation technologies in terms of their general working principles, advantages and shortcomings. AMD treatment technologies can be divided into two major categories, namely prevention and remediation. Prevention techniques mainly focus on inhibiting AMD formation reactions by controlling the source. Remediation techniques focus on the treatment of already produced AMD before their discharge into water bodies. Remediation technologies can be further divided into two broad categories: active and passive. Due to high cost and intensive labor requirements for maintenance of active treatment technologies, passive treatments are widely used all over the world. Besides the conventional passive treatment technologies such as constructed wetlands, anaerobic sulfate-reducing bioreactors, anoxic limestone drains, open limestone channels, limestone leach beds and slag leach beds, this paper also describes emerging passive treatment technologies such as phytoremediation. More intensive research is needed to develop an efficient and cost-effective AMD treatment technology, which can sustain persistent and long-term AMD load.

Effective Ranges of a Modeled Open Limestone Channel

Effective Ranges of a Modeled Open Limestone Channel

English

An Anoxic Limestone Drain (ALD) and Open Limestone Channel (OLC) are primary components of the Project for the Restoration of Rio Juckucha, in Potosi, Bolivia; the goal of which is to improve water quality for irrigation use in the high Andes. The OLC is treating low-pH (2.9) acid mine drainage from a man-made alpine lake receiving discharges from portals, tailings, and waste-rock from active and abandoned mining operations. The ALD is treating milder acid mine drainage (pH 5.4) from an abandoned mine portal. Both systems treat waters that run to Laguna Santa Catalina, which forms the headwaters of Rio Juckucha, a key irrigation water resource. The alkalinity generation and metals removal performance of the recently-constructed ALD and partially-constructed OLC was investigated to determine early-stage treatment characteristics. The results indicated that the OLC at 20% completion was having relatively little impact. The recently-constructed ALD is showing water quality improvements (higher pH and alkalinity). However, there is a concern with increased Zn and Mn concentrations in the ALD effluent. Studies will continue upon construction completion to determine long term performance. Additional

Heavy metal removal of intermittent acid mine drainage with an open limestone channel

This study is focused on the influence of a particular open limestone channel (OLC) on the quality of the surface water drained from an intermittent watercourse. The OLC was constructed along a creek surrounded by upstream tailings deposits, in an extensive, abandoned sulfide-mining site, which generates acidic and heavy metals-rich drainage water during the occasional precipitation that occurs. The overall length of the OLC is 1986 m, it has an average slope of 4.6%, and consists of two main segments. The effectiveness of this channel was evaluated through different physico-chemical parameters: pH, electrical conductivity (EC), total solids (TS), and heavy metal concentrations (Al, Fe, Zn, Ni, Cu, As, Cd, and Pb), measured in surface water. A total of 47 water samples were collected in 12 rainfall events, in the period 2005–2009. Moreover, for three different precipitation events, depletion curves of these parameters were constructed. The values of pH and Ca were increased downstream of the channel, related to the alkalinity and calcium release of the OLC and carbonates present in the watershed, whereas the EC, TS, K, Mg, SO 4 2 - , Al, Mn, Fe, Ni, Cu, Zn, As, Cd, and Sb decreased towards the mouth of the creek. The OLC reduced the input of heavy metals into the Mar Menor lagoon by one order of magnitude. According to the results, this kind of constructive solution is effective with regard to mitigating the effects of intermittent acid mine drainage in Mediterranean and semi-arid regions.

Abandoned Mine Drainage in the Swatara Creek Basin, Southern Anthracite Coalfield, Pennsylvania, USA: 2. Performance of Treatment Systems

A variety of passive and semi-passive treatment systems were constructed by state and local agencies to neutralize acidic mine drainage (AMD) and reduce the transport of dissolved metals in the upper Swatara Creek Basin in the Southern Anthracite Coalfield in eastern Pennsylvania. To evaluate the effectiveness of selected treatment systems installed during 1995–2001, the US Geological Survey collected water-quality data at upstream and downstream locations relative to each system eight or more times annually for a minimum of 3 years at each site during 1996–2007. Performance was normalized among treatment types by dividing the acid load removed by the size of the treatment system. For the limestone sand, open limestone channel, oxic limestone drain, anoxic limestone drain (ALD), and limestone diversion well treatment systems, the size was indicated by the total mass of limestone; for the aerobic wetland systems, the size was indicated by the total surface area of ponds and wetlands. Additionally, the approximate cost per tonne of acid treated over an assumed service life of 20 years was computed. On the basis of these performance metrics, the limestone sand, ALD, oxic limestone drain, and limestone diversion wells had similar ranges of acid-removal efficiency and cost efficiency. However, the open limestone channel had lower removal efficiency and higher cost per ton of acid treated. The wetlands effectively attenuated metals transport but were relatively expensive considering metrics that evaluated acid removal and cost efficiency. Although the water-quality data indicated that all treatments reduced the acidity load from AMD, the ALD was most effective at producing near-neutral pH and attenuating acidity and dissolved metals. The diversion wells were effective at removing acidity and increasing pH of downstream water and exhibited unique potential to treat moderate to high flows associated with storm flow conditions.

Influent Water Quality Affects Performance of Passive Treatment Systems for Acid Mine Drainage

Passive treatment of acid mine drainage (AMD) relies on biological, geochemical, and gravitational processes to neutralize acidity. Published design guidelines use sizing ‘rules of thumb’ based on AMD loadings at design flows. Using average performance data for 82 treatment systems, we used regression modeling to investigate the influence of influent net acidity and water loading on alkalinity generation by five treatment system types. Alkalinity generation increases with influent net acidity loading for all system types. Influent net acidity loading can be deconstructed into concentration and water loading components. In bivariate models, water loading was a predictor of alkalinity generation for all five system types but net acidity was significant only for vertical flow systems (VFs). In multivariate models using both components as performance predictors, both influenced alkalinity generation. These relationships were strongest for anaerobic wetlands (AWs), VFs, and open limestone channels; anoxic limestone drains and limestone leach beds demonstrated these influences less consistently. These results reflect the geochemical mechanisms governing the performance of limestone-based passive treatment system: solubility of limestone decreases as dissolved reaction products and pH increase.

Using pilot trials to test geochemical treatment of acid mine drainage on Stockton Plateau

Herbert Stream, a tributary of the Waimangaroa River on the Stockton Plateau, South Island, New Zealand, has elevated metal concentrations (Al 7.68 ppm, Fe 1.37 ppm, Mn 0.69 ppm, and Zn 0.12 ppm) and low pH (2.3–3.4) characteristic of acid mine drainage. Average flow rate is 5.3 L/s. To determine the effectiveness of different geochemical treatment strategies, small‐scale trials consisting of a reducing and alkalinity producing system (RAPS), a limestone leaching bed (LLB), and an open limestone channel (OLC) were operated for 8 months. All three trial systems performed well, removing metals and raising pH. Maximum removal rates were: Al 99% (all three systems); Fe 97% (RAPS), 99% (LLB), and 95% (OLC); Mn 95% (RAPS), 92% (LLB), and 74% (OLC); and Zn 87% (RAPS) and 91% (LLB). The OLC was less effective than the other trial systems in raising pH, and the effectiveness of Al removal decreased with time, probably due to armouring of the limestone by hydroxide precipitates. Minimal armouring of the limestone in the RAPS and LLB occurred, and the RAPS was successful at reducing oxidised Fe to Fe monosulfides (most likely mackinawite). Based on monitoring of the trial AMD treatment systems, a full‐scale LLB was designed to treat the entire flow of Herbert Stream.

Effectiveness of an Open Limestone Channel in Treating Acid Sulfate Soil Drainage

An open limestone channel (OLC) was constructed within an existing drain to treat the acidic and metal-rich drainage waters generated from an acid sulfate soil (ASS) catchment. The OLC was constructed downstream of a catchment pump and it consisted of a series of ponds and limestone sections. The accumulation of sediment over the limestone, preventing contact of limestone with acidic water, was the greatest problem impacting the OLC in its first year of operation. The continuous or sporadic operation of the catchment pump (at 120 l/s) was not sufficient to flush sediment from the limestone. The accumulation of large amounts of sediment onto the limestone reduced the amount of alkalinity and calcium released into solution. However, if the sediment is removed by agitating the limestone then an equivalent or greater amount of alkalinity may be added to solution and more metals removed from solution compared to fresh limestone. The coating on the limestone had a high concentration of manganese oxides in addition to slightly lower concentrations of aluminium and iron. Removal of these metals from the water was due to the increase in pH produced by limestone dissolution in addition to sorption reactions of the existing coating which had natural microbial activity.

Treatment of acidic water from Bekok River using open limestone channel

Bekok River is one of the raw water resources for Batu Pahat treatment plants which supply treated water for domestic, institutional, commercial and industrial use. Due to soil characteristics and agricultural activities in the catchment area, pH of lower than 5.5 is common to the river, particularly in the stretch where the water intake point is located. However, at times, Bekok River registered low pH of less than 3.5 and hence caused problems to the treatment plants. The cost to neutralise the water increased and treatment scheduling became more complicated. A preliminary laboratory scale study has been conducted to determine the viability of using an open limestone channel as an in-situ pH treatment in the river. Two sets of experiments using synthetic and actual raw waters with various sizes and amount of limestone were carried out. Results indicate the viability of the process. Depending on the initial pH and acidity of the raw water, the pH can rise to 6.0 within 30 minutes of contact time. The amount and size of limestone was found to play an important role in minimising the contact time.

English

State and federal reclamation programs, mining operators, and citizen- based watershed organizations have constructed hundreds of passive systems in the eastern United States over the past 20 years to provide reliable, low cost, low maintenance mine water treatment in remote locations. In 2000, we evaluated 116 systems comprised of eight system types in eight states. We revisited 14 of these sites in 2004 to confirm results from the earlier study. Each system was monitored for influent and effluent flow, pH, net acidity, and metal concentrations. Performance was normalized among types by calculating acid load removed, and also by converting construction cost, projected service life, and metric tonnes of acid load treated into cost per tonne of acid treated. Of the 116 systems, 105 reduced acid load (90%). Average acid load reductions were 0.8 t/yr for Ponds; about 9 t/yr for open limestone channels (OLC), anaerobic wetlands (AnW), aerobic wetlands (AeW), and vertical flow wetlands (VFW); 76 t/yr for slag leach beds (SLB), and about 15 t/yr for limestone leach beds (LSB) and anoxic limestone drains (ALD). Average removal rates ranged from 18 to 2,334 g/day/t for the limestone systems, and 1.7 to 87 g/m 2 /day for the Ponds and wetlands. Average costs for acid removal varied from $36/t/yr for SLB to $1,468/t/yr for Ponds. The 2004 data showed slightly greater removal efficiencies for two Ponds, two VFWs, and one LSB. Large declines in removal were found for one AnW, two VFWs, one ALD, and one OLC. Two OLCs greatly increased efficiency. Most passive systems were effective for >5 yrs, yet there was wide variation in performance within each system type.

Long-term Performance of Passive Acid Mine Drainage Treatment Systems

State and federal reclamation programs, mining operators, and citizen-based watershed organizations have constructed hundreds of passive systems in the eastern U. S. over the past 20 years to provide reliable, low cost, low maintenance mine water treatment in remote locations. While performance has been reported for individual systems, there has not been a comprehensive evaluation of the performance of each treatment type for a wide variety of conditions. We evaluated 83 systems; five types in eight states. Each system was monitored for influent and effluent flow, ph, net acidity, and metal concentrations. Performance was normalized among types by calclating acid loading reductions and removals, and by converting construction cost, projected service life, and metric tonnes of acid load treated into cost per tonne of acid treated. Of the 83 systems, 82 reduced acid load. Average acid load reductions were 9.9 t/yr for open limestone channels (OLC), 10.1 t/yr for vertical flow wetland (VFW), 11.9 t/yr for anaerobic wetlands (AnW), 16.6 t/yr for limestone leach beds (LSB), and 22.2 t/yr for anoxic limestone drains (ALD). Average costs for acid removal varied from $83/t/yr for ALDs to $527 for AnWs. Average acid removals were 25 g/m2/day for AnWs, 62 g/m2/day for VFWs, 22 g/day/t for OLCs, 28 g/day/t for LSBs, and 56 g/day/t for ALDs. It appears that the majority of passive systems are effective but there was wide variation within each system type, so improved reliability and efficiency are needed. This report is an initial step in determining passive treatment system performance; additional work is needed to refine system designs and monitoring.

English

Acid mine drainage (AMD) occurs when metal sulfides are exposed to oxidizing conditions. Leaching of reaction products into surface waters pollute over 20,000 km of streams in the USA alone. Mining companies must predict the potential of creating AMD by using overburden analyses. Where a potential exists, special handling of overburden materials and quick coverage of acid-producing materials in the backfill should be practiced. The addition of acid-neutralizing materials can reduce or eliminate AMD problems. Placing acid-producing materials under dry barriers can isolate these materials from air and water. Other AMD control technologies being researched include injection of alkaline materials (ashes and limestone) into abandoned underground mines and into buried acid material in mine backfills, remining of abandoned areas, and installation of alkaline recharge trenches. Chemicals used for treating AMD are Ca(OH)2, CaO, NaOH, Na2CO3, and NH3, with each having advantages under certain conditions. Under low-flow situations, all of the chemicals except Ca(OH)2 are cost effective, while at high flow Ca(OH)2 and CaO are clearly the most cost effective. Floc, the metal hydroxide material collected after treatment, is disposed of in abandoned deep mines, refuse piles, or left in collection ponds. Wetlands remove metals from AMD through formation of oxyhydroxides and sulfides, exchange and organic complexation reactions, and direct plant uptake. Aerobic wetlands are used when water contains enough alkalinity to promote metal precipitation and anaerobic wetlands are used when alkalinity must be generated by microbial sulfate reduction and limestone dissolution. Anoxic limestone drains are buried trenches of limestone that intercept AMD underground to generate alkalinity. Under anoxia, limestone should not be coated with Fe +3 hydroxides in the drain, decreasing the likelihood of clogging. Successive alkalinity-producing systems pre-treat oxygenated AMD with organic matter to remove oxygen and Fe +3 , and then the water is introduced into limestone underneath the organic matter. Open limestone channels use limestone in aerobic environments to treat AMD. Coating of limestone occurs and the reduced limestone dissolution is designed into the treatment system. Alkaline leach beds, containing either limestone or slag, are used to add alkalinity to acid water. At present, most passive systems offer short-term treatment, and are more practical for installation on abandoned sites or watershed restoration projects where effluent limits do not apply and where some removal of acid and metals will benefit a stream.

EVALUATION OF A LIMESTONE CHANNEL AND WETLAND SYSTEM FOR TREATING ACID MINE DRAINAGE

The Carpentertown Coal and Coke Company operated two drift mines on the site for 17 years closing in December 1987, but the company continued to operate a cleaning plant and coal refuse disposal site until the company declared bankruptcy in June 1989. In summer of 1993, eight acid seeps developed from the 2 ha coal refuse site with a combined flow of 36 1/min with iron and manganese loading rates of 419 and 576 gm/day. In 1995, a 212 m (700 ft) open limestone channel (OLC) and a 344 m{sup 2} (1,142 ft{sup 2}) and a 2,110 m{sup 2}(43,750 ft{sup 2}) aerobic wetland was constructed as a passive treatment system. over the 34-month monitoring period, the acid loading to the receiving stream was reduced by 88% with a corresponding increase of 111% in alkalinity. The iron and manganese loading to the receiving stream was reduced by 91% and 57%, respectively.

Acid Mine Drainage Treatment with Armored Limestone in Open Limestone Channels

Much attention has been devoted to developing inexpensive, limestone-based systems for treating acid mine drainage (AMD) with little or no maintenance. Treatment of AMD with limestone results in a surface coating of metal hydroxides, a process known as limestone armoring. Once armored, limestone is assumed to cease dissolution and acid neutralization. Laboratory and field experiments determined acidity changes in AMD when contacted by armored and unarmored limestone and investigated the implications of armoring on the construction of open limestone channels for treating AMD. Results of a laboratory titration study indicated armored limestone was only 2 to 45% less effective in neutralizing a hydrochloric acid solution as unarmored limestone. A laboratory container study showed that armored limestone was 90% as effective in neutralizing AMD as unarmored limestone. The field study surveyed 2- to 8-yr-old, rock-lined channels constructed of sandstone or limestone, and measured water quality changes down the length of the channel. Open limestone channels, though armored, reduced more acidity in AMD (4-62%) than the sandstone channel (2%). The results from open limestone channels were compared to an acid neutralization kinetics model that predicts the rate of acid neutralization for a specified channel size, and AMD flow and acidity concentration. The open limestone channels in the field neutralized more acidity than the model predicted. Open limestone channels show promise for neutralizing AMD in watershed restoration projects and abandoned mine land (AML) reclamation projects where one-time installation costs are incurred, little to no maintenance is required, and systems do not have to meet specific water quality standards.