Artificial Swimming Lagoons Guidance

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Artificial swimming lagoons have emerged as a new and innovative way to enjoy recreational water activities. These large and uniquely designed aquatic venues pose distinct challenges that cannot be addressed in the same manner as a typical swimming pool. Several of these venues are already present in the United States, with more planned for future construction.

This document addresses the needs of the aquatic sector and environmental public health practitioners responsible for responding to health and safety issues by providing recommendations for these unique venues. This guidance is based on science and best practices.

This guidance addresses the design, construction, operation, and management recommendations that are specific for artificial swimming lagoons. The recommendations ensure proper filtration, disinfection, and management of the water. The recommendations also identify ways to reduce the risk of illness and injury, including drowning. Unless addressed below, please refer to CDC’s Model Aquatic Health Code (MAHC) for guidance on the design, construction, operations, and management of typical public swimming pools and traditional aquatic venues.

Definitions

An artificial swimming lagoon (ASL) is a large-format aquatic venue. The ASL is designed to mimic a natural water body and consists of one or more designated swimming areas with the remainder of the lagoon designated for water sports and non-traditional uses, including:

  • Kayaking
  • Paddle-boarding
  • Windsurfing
  • Boating
  • Scuba diving training

A designated swimming area (DSA) is a zone of water within an ASL used for swimming, wading, or bathing. Typically, a DSA is visually separated from the rest of the water sports area using a rope and float line or similar device approved by the authority having jurisdiction (AHJ). Constant flow of treated water into DSAs ensures water quality. The flowing water displaces potentially contaminated water from the DSAs, creating a positive hydraulic gradient from DSAs to the water sports area (Figure 1). Water is removed from the water sports area and treated before it is returned to DSAs. Typically, DSAs are shallower than the open-water areas used for boating, kayaking, and paddle-boarding. Swimming, wading, and bathing are not permitted outside of DSAs.

DPD- FC (L2 – N,N-diethyl-p-phenylenediamine-Free Chlorine) is the free chlorine (FC) or free available chlorine (FAC) concentration from DPD-based test methods. The DPD-based test result for FC includes cyanurate-bound available chlorine (CBC) as well as hypochlorous acid (HOCl) and hypochlorite (OCl–). That is, DPD-FC = free available chlorine (FAC) plus CBC. The terms DPD-FC and FAC would be interchangeable only in the absence of cyanuric acid.

Figure 1. ASL Water Flow Schematic
Image courtesy of Crystal Lagoons

ASL Water Flow Schematic
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ASL Design and Construction

Most likely, an ASL will deviate from the required provisions for typical pools. Some AHJs might require a plan to be submitted and reviewed, or they might require variances. A plan review or variances might also be required for any substantial renovations to an ASL.

CDC recommends following these ASL guidelines, where not in conflict with AHJ provisions:

  • Address health and safety concerns generated by variances.
  • Demonstrate, in the design and construction documentation, that an ASL is within the limits of sound engineering practice and achieves or exceeds the equivalent health and safety provisions required for pools.
    • Include computational fluid dynamics (CFD) analysis to document a positive hydraulic gradient from DSA(s) to the remainder of the ASL and the absence of dead zones in DSAs.
  • Use NSF/ANSI 50 certified equipment or an approved alternative that meets or exceeds NSF standards.
  • Use filters and filter media that are certified, listed, and labeled as NSF/ANSI 50 by an ANSI-accredited certification organization, or approved alternative meeting or exceeding NSF standards.
  • Set a minimum performance goal for ASL filters to remove Cryptosporidium oocysts at least 90% (1 log) per single pass.
  • Use chemicals that are NSF/ANSI 50 certified or have EPA approval for use in recreational water.

Technical Specifications

Peak Occupancy
  • Peak occupancy for each DSA is not to exceed the total (cumulative) surface area in square feet of the ASL’s individual DSA, divided by the density factor (D) = 20ft2 (1.86m2) per bather.

Materials

  • ASL construction should incorporate a containment system using impervious material(s), which provides a smooth and slip-resistant, easily cleaned, watertight structure capable of withstanding the anticipated stresses for full and empty conditions. Wood, sand, or earth are not permitted as an interior finish. Construction and design personnel should take into consideration climatic, hydrostatic, and seismic conditions.
  • The floors and walls of the containment system should be white or light pastel in color such that the following items can be identified on the bottom:
    • A person or body submerged in the water
    • Algae growth
    • Debris or dirt
    • Cracks in the surface finish
  • Light pastel colors should be consistent with a Munsell color value 6.5 or higher.
  • Design and construction should assure the bottom of the ASL is visible in all areas.

Structure

The design of the ASL shape needs to provide for swimmer safety, assure easy cleaning and maintenance, and support effective supervision and surveillance of bathers and patrons. The following design considerations are recommended:

  • DSAs with water depths less than 3 feet (0.9 m), the slope of the floor should not exceed 1 foot (30.5 cm) vertical drop for every 12 feet (3.7 m) horizontal.
  • In water depths greater than 3 feet (0.9 m), the slope of the floor of the DSA should not exceed 1 foot (30.5 cm) vertical drop for every 10 feet (3.3 m) horizontal.
  • Outside of a DSA, the floor slope of an ASL should not exceed 1 foot (30.5 cm) vertical drop for every 3 feet (0.9 m) horizontal.

DSA Boundaries

  • DSAs can have a single shore entry, depending on design. The remaining border of the DSA will be outlined in the water, like a beach setting. The shore can serve as the only wet deck.
  • For each DSA, mark designated boundaries in the water, or for any other aquatic feature accessible by bathers or patrons, with a rope and float line or similar device(s) approved by the AHJ.
  • Rope line floats, or buoys, should be at regular intervals no further than 25 feet apart and where lines are joined.
  • Provide clearly visible depth marker buoys or a similar device approved by the AHJ on the boundary float line and indicate the maximum depth within the DSA.

DSA Access and Egress

  • Where zero-depth entry is not used along the entire entrance of each DSA, there should be at least two means of access and egress:
    • Stairs
    • Handrails
    • Grab rails
    • Recessed steps
    • Ladders
    • Zero depth entries
  • For DSAs wider than 30 feet (9.1 m), such means of access/egress shall not be more than 75 feet (22.9 m) apart.
  • Permanent or portable steps, ramps, handrails, lifts, or other devices designed to accommodate persons with disabilities must be provided as required by the Americans with Disabilities Act. Lifts mounted into the wet deck should have a minimum 4-foot-wide walkway behind the mount, on the shore or deck behind the lift. Compliance with the Americans with Disabilities Act might require multiple means of access/egress for those with disabilities.
  • The recommendations for swimming access and egress do not apply outside of DSAs, in the water sports area of the ASL.
  • Trench drains are not required along zero depth entries at the waterline.

DSA Zero Depth Entries

  • Where zero depth entries are provided, they should be constructed with slip-resistant materials.
  • The floor slope for zero depth entries should have a maximum grade that does not exceed 1 foot (30.5 cm) vertical drop for every 12 feet (3.7 m) horizontal. The floor slope does not need to be consistent throughout the DSA. Variability in floor slope(s) is acceptable, as long as the 1:12 ratio is not exceeded.
  • Zero depth entry deck areas may slope toward the water for no more than 7 feet (2.133 m), as measured from the water’s edge outward (away from the water). Beyond 7 feet from the waterline, the deck or other surfaces should slope away from the lagoon at a minimum of 1:50 ratio to a maximum of 1:25 ratio.
  • Docks for aquatic activities such as sailing or kayaking, located outside of DSAs, are exempt from guidance for zero depth entries.
  • Zero-depth entry decks should be extended out at least 4 feet from the edge of the waterline.

DSA Depth Markers and Markings

  • Where applicable, depth markers are recommended on the inside vertical walls of a DSA. These markers or signs are recommended only along the accessible perimeter of the DSA.
  • Signage may be substituted for markers if approved by the AHJ.
  • Markings should use materials that will not fade over time.
  • Follow the guidance below for the use of markers and signage:
    • Use depth markers on a sign at access points indicating the deepest point in the DSA.
    • Install NO DIVING markers where access to a portion of the DSA with a vertical wall is not blocked or obstructed by an approved enclosure or barrier.
    • For water depths 5 feet (1.5 m) or shallower, all depth markers should have warning signs with the words NO DIVING, along with the universal international symbol for NO DIVING.
    • Space NO DIVING warning signs and symbols at no more than 25-foot (7.6 m) intervals around the DSA perimeter edge.
    • Use NO DIVING markers constructed of a durable material resistant to local weather conditions.
    • Ensure all NO DIVING lettering and symbols are at least 4 inches (10.2 cm) in height.

DSA Recirculation and Water Treatment

  • Operators should ensure the ASL recirculation and filtration system provides circulation, filtration, skimming, and disinfection of the water to maintain the jurisdiction’s requirements for pool water quality and clarity in each DSA.
  • Alternatives to the design and operational pool requirements are allowed in ASL. The design professional must provide a rationale and include a CFD analysis.
  • Turnover time concepts for typical pools do not apply to ASLs overall. Operators should ensure that water in each DSA is replaced at a renewal rate of 6 hours or less.
  • During the times of year when DSAs are open for bathers, an operator should maintain a positive hydraulic gradient. Using CFD or other means, the design professional should ensure the volumetric replacement of water in each DSA occurs every 6 hours or less.
  • Maintain replacement DSA water through the continuous operation (24/7) of the recirculation system and flow direction of inlet nozzles.
  • Recirculation and disinfection systems should be able to treat water either under gravity feed or via a pressure system as justified by the design professional. Operators need to ensure that water for the DSA is pulled into the filtration and disinfection system, then returns to the DSA and displaces water into the water sports area.

ASL Water Supply/Wastewater Disposal

  • Water serving an ASL should be supplied from a potable water source or other source approved by the AHJ. Salt electrolytic chlorine generation is permitted.
  • Wastewater from an ASL, including filter backwash water, can be reused in the ASL, if discharged into the water sports area(s) and not directly into DSAs (at least 50 feet from a DSA boundary).
    • If backwash water is to be re-used, it first is to be
      • released to a sedimentation tank with flocculant and allowed to settle,
      • re-filtered, and
      • disinfected so that DPD-FC concentration of re-use backwash water is a minimum of 1.0 ppm (mg/L).
    • Additionally, wastewater can be discharged to a sanitary sewer system having sufficient capacity to collect and treat wastewater, or to an on-site disposal system designed for this purpose.
    • If an external pollution source that is affecting the water quality of the lagoon is identified, the entire ASL is to be closed for use immediately. Prior to re-opening, the source should be eliminated and all water remediated and tested for compliance with quality parameters.

Decks and Equipment

  • Shoreline decks should be extended out at least 4 feet from the edge of the waterline.
  • Decks are not applicable for the water borders or boundaries (rope and float line) of a DSA.
  • An island that is designed for bather traffic may be accessible by a zero-depth entry. Deck requirements for a pool are not applicable to an island within an ASL. Depth markings and warning signs for islands should be consistent, as listed above.
  • Each DSA should have a means to limit entry into the deck and beach area, to deter (barrier) and prevent (enclosure) unauthorized entry. Use a self-closing and self-latching gate to DSA beach areas.
  • Barriers or enclosures are not needed immediately adjacent to the water sports area.
  • Decks are not needed adjacent to the water sports area.

Hygiene Facilities

  • Hygiene facilities and hygiene fixtures should be provided. The minimum number, types, and locations of hygiene facilities will be based on the peak occupancy of each DSA as described above. For the water sports area, the minimum number, types, and locations of hygiene facilities will be based on the AHJ’s requirements for ambient or natural water bodies.
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ASL Operation and Management

Swimming in a DSA should only be allowed when operators are able to maintain the positive pressure gradient of treated water flowing into and displacing potentially contaminated water out of the DSA (Figure 2). When an ASL is permanently shut down, owner/operators should ensure that the enclosure restricts all unauthorized entry to the property from the public. Work with the AHJ to develop a plan for decommissioning or draining all water from the entire ASL. When draining is not allowed due to discharge and flooding concerns, determine whether the water can be re-purposed.

Figure 2. Designated Swimming Area Water Flow Schematic

Designated Swimming Area Water Flow Schematic

DSA Water Quality

When the area is open for bathers, water quality in the DSA should always meet the criteria in Table 1.

Table 1: Water Quality in Designated Swimming Areas

Parameter Value
Table 1: Water Quality in Designated Swimming Areas
DPD-FC 1.0–10.0 ppm (mg/L)
Bromine 3.0–8 ppm (mg/L)
CYA* ≤ 15 ppm
pH 7.0–7.8
Turbidity ≤0.5 NTU

*If a stabilizer is used, this concentration allows for immediate response to fecal incidents based on CYA guidelines found in CDC’s MAHC 6.5 Fecal/Vomit/Blood Contamination Response.

Clarity

  • Outside of a DSA, the water should be sufficiently clear such that the bottom is visible while the water is static.
  • Turbidity can be monitored using an NSF/ANSI 50 certified online turbidimeter that reads influent clarity.
  • When a facility is closed and not in use (such as during the off season), clarity should be maintained in all DSAs and the water sports area at ≤0.5 nephelometric turbidity units.
  • If turbidity is evaluated manually, then the following criteria should be met:
    • In DSAs with a water depth of 10 feet or less, the water clarity should be sufficient such that a 4-inch by 4-inch (10.2 cm x 10.2 cm) black square, Secchi Disk, or marker tile of contrasting color, is visible on the floor at the deepest part of the DSA. The marker tile should be clearly and immediately seen by an observer on the water surface above the marker or at any point on the shore/deck up to 30 feet (9.1m) away in a direct line of sight from the tile.
    • For DSAs over 10 feet (3.0 m) deep, the water clarity should be sufficient such that an 8-inch by 8-inch (20.3 x 20.3 cm) black square, Secchi Disk, or marker tile of contrasting color, is visible on the floor at the deepest part of the DSA. The marker tile should be clearly and immediately seen by an observer on the water surface above the marker or at any point on the shore/deck up to 30 feet (9.1m) away in a direct line of sight from the tile.

Testing Equipment

  • Water quality should be monitored and controlled in each DSA using an NSF/ANSI 50 certified water chemistry control system. Consider using real-time remote communications and alert notification capabilities.
  • If remote chemical monitoring sensors are used, one chemical sensor should be installed in each DSA, in addition to waterside manual testing. Remote systems can be used to aid the monitoring of ASLs and DSAs, but not as a substitute for on-site operation and maintenance.
  • Algae growth, particularly on the venue floor, is to be prevented in both the DSAs and the water sports area. However, recommendations for water sports area quality parameters in Table 1 are not applicable.
  • During non-use, water cannot be left to become contaminated and harmful, or capable of producing unpleasant odors.

Water Testing Plans

Designated Swimming Areas – Measurements for sanitizer concentration (free available chlorine or bromine) and pH should be taken each day prior to opening to bathers in each DSA. The operator should ensure that the sanitizer concentration and pH are in the proper range before allowing bathers into the water. Additional tests for sanitizer levels and pH should be made at least two times a day in each DSA.

Water Sports Area – Water in the water sports area is quite different than natural lakes, rivers, and beaches. This constructed environment will not allow aquatic wildlife habitation, nor will it contain a natural sediment/bottom with aquatic plants. Contamination detected through fecal indicator bacteria monitoring could be human or animal. However, contamination risks are much different in an ASL than in other natural waters with non-human fecal inputs like lakes and beaches.

Boaters and kayakers do have some contact with the water in water sports areas, but not in the same manner as swimmers in DSAs. All water in the ASL is being displaced from the sanitized DSA to the water sports area of the lagoon, through a positive hydraulic gradient. Therefore, given the significant dilution of any potential contaminants in the water sports area, all water in the ASL can be expected to be clear and suitable for recreational use.

Microbial Water Quality Monitoring

  • Microbial water quality testing is not recommended in treated DSAs, unless a DSA is suspected of being associated with an outbreak. ASL operators should develop a microbial water quality monitoring plan for the water sports area and establish minimum microbiological indicators in the ASL water sports area when open to the public.
  • Microbial numbers could be expected to be lower than natural waters because of the built-environment and recent treatment in the DSA.
  • Refer to local, state, territorial, and tribal laws for recreational water quality standards for natural bodies of water. Jurisdictions might already have guidance on how to address water quality indicators for freshwater lakes, rivers, and beaches with recreational water access.
  • In the absence of scientific data, natural water body monitoring plans could be applied to microbial monitoring in the water sports area.
  • The U.S. Environmental Protection Agency (EPA) has resources on developing site-specific water monitoring plans for non-primary contact (e.g., canoeing, kayaking) that are based on potential fecal sources (e.g., human, bird, racoon, dog). EPA also has published recommendations for monitoring freshwater beaches for safe recreational use.

For example, the state of Florida requires the following for non-swimming areas in ASLs:

  • Tests for bacteriological levels in non-swimming areas should be performed at least once monthly.
  • A set of two samples should be collected for every 500 feet of shoreline outside of any DSA.
  • The samples should be taken a foot below the surface in at least 3 feet of water and at least 25 feet apart.
  • Owners and operators of an ASL can test for either enterococci or E. coli bacteria.
  • The enterococci density should not exceed 61 colony forming units (CFU) per 100 mL of water in any single sample.
  • E. coli density should not exceed 235 CFU per 100 mL of water in any single sample of water.
  • If the test results of these samples exceed the values listed above, the owner/operator should notify the AHJ within 24-hours of receiving the results, and the owner/operator should resample within 24 hours.
  • All sampling results should be submitted to the AHJ.
  • If resampling cannot be completed within 24 hours, or in the event the resampling results continue to exceed the bacteriological values listed above, the owner should immediately close the ASL to patrons.
  • The ASL should not be reopened until additional testing reveals the water meets water quality parameters.

For example, the state of North Carolina requires the following for non-swimming areas in ASLs:

  • Tests for bacteriological levels in non-swimming areas should be performed at least once weekly.
  • One sample should be collected for every 250 feet of shoreline outside of any DSA, with no more than 300 feet and no less than 25 feet between any two sampling locations.
  • The samples should be taken a foot below the surface in at least 3 feet of water.
  • NC references their natural body of water recreational swimming standard (lakes and beaches) for the bacterial counts in water sports and will close if the samples exceed that standard.
  • The enterococcus level shall not equal or exceed either: (1) a geometric mean of 35 enterococci per 100 milliliters of water or (2) a single sample of 104 enterococci per 100 milliliters of water.

ASL Safety Standards

  • Swimming at night in DSAs is not recommended.
  • Do not allow watercrafts powered by internal combustion engines in ASLs.
  • Do not use vacuum systems in a DSA while the area is open for swimming.
  • Provide certified lifeguard supervision in DSAs.
  • Provide U.S. Coast Guard-approved personal floatation devices or life jackets to participants in the water sports area.
  • Refer to local, state, territorial, and tribal laws for requirements on the use of personal floatation devices for minors while on watercrafts.
  • Additional lifeguard and safety equipment recommendations for DSAs can be found in CDC’s Model Aquatic Health Code, chapter 5.8.5 (Lifeguard and Safety-related Equipment).

Fecal Incident Response

Formed-Stool Contamination in a DSA
  • Have all bathers leave the DSA immediately.
  • Remove the formed/solid stool from the swimming area with a net and proper PPE.
  • Check the DPD-FC concentration in the contaminated DSA.
    • Ensure that DPD-FC concentration of water exiting the inlet nozzles is a minimum of 2 ppm (mg/L).
    • Make necessary chemical adjustments through use of the directional floor inlets in the contaminated DSA.
  • Keep all bathers out of the contaminated DSA for at least an additional 60 minutes, before allowing re-entry.
Diarrheal-Stool Contamination in a DSA
  • Have all bathers leave the DSA immediately.
  • Check the DPD-FC concentration in the contaminated DSA.
    • Ensure that DPD-FC concentration of water exiting the inlet nozzles is a minimum of 20 ppm (mg/L).
    • Make necessary chemical adjustments through use of the directional floor inlets in the contaminated DSA.
  • Keep all bathers out of contaminated the DSA for at least an additional 6 hours, before allowing re-entry.
    • Or keep all bathers out of the contaminated DSA for enough time until a complete water replenishment/displacement cycle of DSA water has occurred, as demonstrated by CFD.

Resources

  • Florida Building Code, Section 454.1 – Public Swimming Pools and Bathing Places.
  • North Carolina Health Code, 15A NCAC 18A .2543 – Water Recreation Attractions.
  • S. EPA – 2012 Recreational Water Quality Criteria (Office of Water 820-F-12-058).
    • Provides recommendations for both primary contact recreators (swimmers) and non-primary contact recreators (boaters and kayakers), as well as additional information for use by states in developing or implementing safe freshwater recreational programs.
  • EPA guidance document to apply the 2012 criteria to non-primary contact exposure scenarios (Office of Water 823-B-22-001).
  • EPA National Beach Guidance and Required Performance Criteria for Grants, 2014 Edition (EPA-823-B-14-001).
  • Texas Health Code, Title 25, Part 1, Chapter 265, Subchapter K – General Sanitation, Artificial Swimming Lagoons.
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