**1. Introduction**

Agricultural runoff contains excess quantities of diverse pollutants, such as sediments, nutrients, pathogens, veterinary medicines, pesticides, and metals. Modern agriculture heavily relies on agro-chemicals, such as pesticides, herbicides, and hormones, that would gran<sup>t</sup> a greater yield in a shorter period [1]. As the demands for food have increased, so has the intensity of agricultural activities and animal feed operations [2]. As a result, agricultural practices over the past years have included more pesticides and inorganic fertilizers [3,4]. Carvalho and colleagues (1997) reported that North American farmers relied on herbicides 43.3% of the time, while European farmers used it slightly less at 26.3% in 1993 [5]. In 2005, there were more than 800 newly registered pesticides in the European Union [6]. Additionally, approximately two million tons of pesticides were used globally in 2019, with China and the USA being the two major users [7].

These chemicals are perfect for increasing yield but are ecologically detrimental when they leave agricultural ecosystems in runoff water following storms [8]. Studies

**Citation:** Tang, Z.; Wood, J.; Smith, D.; Thapa, A.; Aryal, N. A Review on Constructed Treatment Wetlands for Removal of Pollutants in the Agricultural Runoff. *Sustainability* **2021**, *13*, 13578. https://doi.org/ 10.3390/su132413578

Academic Editors: Muhammad Arslan, Muhammad Afzal and Naser A. Anjum

Received: 12 October 2021 Accepted: 6 December 2021 Published: 8 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

have shown that only 1% of pesticides applied to crops are effective, the other 99% enter the atmosphere, soils, and bodies of water through non-targeted contamination [9]. In livestock production, animal waste can act as reservoirs for antibiotic resistant genes (ARGs) and antibiotic resistant bacteria (ARBs) [10–12]. Another study found the prevalence of veterinary pharmaceuticals to be higher in soil than in water, indicating likeliness of movement to water resources through agricultural runoff [13].

In the long run, these chemicals have the ability to negatively impact food security and agricultural sustainability [14]. Termed chemicals of emerging concern (CECs), these compounds have long been a threat for water quality. According to the Environmental Protection Agency (EPA), CECs include but are not limited to nanoparticles, pharmaceuticals, personal care products (PCPs), estrogenic compounds, flame-retardants, detergents, and other industrial chemicals. All of these contaminants, many of which have agricultural origin, significantly influence human health and aquatic life [15].

Treatment of diffuse source pollution, such as agricultural runoff, requires a lowcost, passive, and nature-based approach known as an ecological engineering approach. Constructed wetland (CW) is a natural ecological alternative to the conventional methods for treating various types of wastewater, including agricultural runoff [16]. The EPA (1993) defines CWs as engineered systems that are designed and constructed to utilize natural processes [17]. Specially designed CWs could be used to treat wastewater in a system that mimics their natural components. The use of wetland plants to treat wastewater is a technique that was firstly studied in the 1950s by German scientist Dr. Ka the Seidel; since then, the idea has expanded greatly and is a very sustainable way of naturally treating many sources of wastewater [18]. CWs are more beneficial than conventional wastewater treatment methods because they require lower energy and less operational effort, but they are also land intensive [16]. CWs are versatile in their functioning, serving as a tool for water quality improvement, hydrological buffers, reservoirs, and nature development/recreational areas [19]. Through imitation of natural wetland systems, such as marshes with wetland plants, soils, and soil microorganisms, CWs are capable of removing diverse contaminants from different wastewater sources [20].

However, there is still very little known about the biotic and abiotic influences and interactions that allows this treatment of water and soil to take place [17]. While much of the previous reviews focused on how CWs are used to efficiently remove nutrients, such as nitrogen and phosphorus, and sediments from wastewaters [21,22], this paper focuses on the occurrence of pollutants in the agricultural runoff and how this cost-effective green approach [23] can be used to remove pollutants from agricultural runoff for mitigation of the negative environmental impacts of agricultural intensification. Focus pollutants include veterinary medicines, antimicrobial resistant genes, insecticides, herbicides, and pesticides.

#### **2. Approach and Definitions**

In this article, we reviewed global literature that focused on CWs used for the treatment of agricultural runoff or wastewater and the characteristics of their design. Scholarly databases were searched using keywords, such as constructed wetlands, agricultural runoff, ARGs, ARBs, pesticides, veterinary antibiotics, chemicals of emerging concern, and their combination to source relevant articles, reports, books, and conference proceedings published in recent years. Both lab-scale and field-scale experiments that studied effective removal rates of contaminants in these systems were considered. The search resulted in over 60 publications that were examined and subsequently summarized in this article directly or indirectly.

CWs are generally classed based on the life form of the dominating large aquatic plant or macrophyte in the system [24] or water-flow regime [25]. Figure 1 shows the classification of CW and their characteristics, which includes flow and flow direction [18,25,26]. Search results were screened based on their relevancy to include CWs that were subsurface horizontal flow (SSHF), subsurface vertical flow (SSVF), surface flow (SF) and hybrid and

were used to remove contaminants that were not nutrients (nitrogen (N), phosphorous (P), total nitrogen (TN), total phosphorous (TP), and sediment).

**Figure 1.** Classification of constructed wetlands.

Hydrological factors dictate the functioning of wetlands as they are directly linked to the ecosystem's biotic and abiotic processes. These processes are what influences both the biological (nutrient availability, microbial community, plant community) and physicochemical (soil pH, water pH, oxidation-reduction potential (ORP)) parameters in CWs [27]. Success of CWs is heavily dependent on the hydraulic residence time (HRT) [28, 29] and the hydraulic loading rate (HLR) [30,31]. Various factors, such as wetland design, scale, size, water depth, HRT, HLR, substrate, experiment duration, source of pollutant, pollutant influent concentration, removal percentage, and major mechanisms responsible for removal of pollutants were tabulated, represented in graphs, or analyzed further.

#### **3. Occurrence of Pollutants in Agricultural Runoff**

Diverse pollutants have been measured in agricultural runoff. Pesticides, herbicides, and veterinary pharmaceuticals are present in agricultural runoff and are major threats to water quality health [32]. Concentrations of CECs have been found in quantities in excess of 0.01 mg/L, especially during storm events [33]. The antibiotics found mostly in agricultural runoff from the reviewed articles are mainly tetracyclines, sulfamonomethoxine, enrofloxacin, and trimethoprim, which are either used for disease prevention or as growth promoters in the industry [34–41]. A Chesapeake Bay study found high concentrations of antibiotics (azithromycin (AZI), clarithromycin (CLA), difloxacin (DIF), enrofloxacin (ENR), norfloxacin (NOR), roxithromycin (ROX), and sulfamethoxazole (SMX)), and hormones (mainly estrogen derivatives) due to wastewater effluents and agricultural runoff [33]. Antibiotics in both swine and dairy cattle farm effluents were found at high concentrations in China, which implies frequent application of these antibiotics during the production process [42]. Since China is one of the largest producers of animals in the world, significant consumption and release to the environment are expected.

Animal husbandry is a major source of environmental ARGs and ARBs [12]. ARG dissemination from flowing water normally happens from ground or surface water sources receiving effluents from domestic, municipal, and agricultural sources, such as livestock farms [43,44]. Through horizontal gene transfer, bacteria are able transfer resistance from one organism to the other. Oliver and colleagues studied dairy manure systems and found the presence of bacteria, such as *Enterobacteriaceae* (specifically nontyphoidal *Salmonella*), antibiotic-resistant *Campylobacter*, methicillin- and vancomycin-resistant *Staphylococcus*, and vancomycin-resistant *Enterococcus,* which the Centers for Disease Control and Prevention (CDC) deemed clinically dangerous to be prevalent. Additionally, they also found that some of these bacteria were able to resist up to five antibiotics [45]. A Chinese study (2018) found 18 types of ARGs from swine feedlots in the surrounding environment, namely streams and agricultural soils [46]. Genes dominant in swine manure were found to be those that were resistant to tetracycline (TC), aminoglycoside (AGR), chloramphenicol (CPR), multidrug (MDR), sulfonamide (SNR) and beta-lactam (BLR) [46–59]. SNR genes were also found abundantly in dairy manure storm runoffs [60]. Background bacterial DNA concentrations were indicated by 16S rRNA data as high as 4.10 × 10<sup>13</sup> copies/mL [61].

The occurrence of pesticides was also found to be prevalent in agricultural runoff effluent [62–65]. A Mexican study found priority pollutants, such as endosulfan, an insecticide that is authorized for use in the country, to be in excess of 8.656 × 10−<sup>3</sup> mg/L in runoff water during storms [66]. Other major contaminants in agricultural runoff include veterinary pharmaceuticals and personal care products (PPCPs), such as naproxen, estrone (and other estrogenic derivatives), which are used mainly for pain suppression or growth enhancement for animals [65–69].

A summary of occurrence of these pollutants in the agricultural wastewater (Figure 2) indicate presence of greater than 1000 mg/L biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total suspended solids (TSS); sub part-per-trillion to 30 part-per million of antibiotics, hormones, and veterinary pharmaceuticals, and up to 4.1 × 10<sup>12</sup> cells per mL of bacteria [42,65,70–72]. Herbicides were found to be more dominant in the agricultural runoff as it had been found as high as 530 mg/L. The prevalence of other contaminants, such as metals and fungicides, were much lower than the other CECs considered [42,66,68,70].

**Figure 2.** Occurrences of pollutants in the agricultural runoff: (**a**) Total suspended solids, biochemical oxygen demand, and chemical oxygen demand, (**b**) metals, fungicides, herbicides and pesticides, (**c**) antibiotics, hormones and veterinary pharmaceuticals, (**d**) antibiotic resistant bacteria and genes, and bacteria. Data are mean ± standard deviation. Note logarithmic y-axis.

#### **4. Removal of Pollutants by Wetlands and Processes for Their Removal**

Many studies have been conducted on the applications of constructed wetlands to remove pollutants from agricultural runoff and wastewater. Based on the study scales, this section has been divided into lab-scale and field-scale for further discussion.
