Question 1

What is the dry side of Hawaii?

Accepted Answer

The dry side (leeward, kona) is the coast sheltered from prevailing northeast trade winds — typically the south or west side of each island. Mountains intercept moisture-laden trade winds; rainfall falls on the windward side and the leeward side stays dry. Hawaiʻi's driest coasts get under 15 inches annually (Kohala Coast on the Big Island, South Maui's Wailea-Mākena, Po‘ipu on Kauaʻi) while windward valleys can exceed 200 inches (Hāna, Mount Waiʻaleʻale on Kauaʻi). The rainfall difference creates a roughly 10–100× difference in stream-driven beach contamination between sides.

Question 2

Which side of each Hawaiian island has the cleanest beach water?

Accepted Answer

Big Island: Kohala Coast (west/leeward) is the cleanest in Hawaiʻi — Hāpuna, Mauna Kea Beach, Anaehoʻomalu Bay all consistently test at 0/30 days under advisory in DOH data. The windward Hilo side is the wettest area in Hawaiʻi and tests dramatically dirtier. Maui: South Maui (Wailea, Kihei, Mākena) and West Maui (Kāʻanapali, Kapalua) — both leeward — test much cleaner than the windward Hāna coast. Oʻahu: West side (Ko Olina lagoons) is the cleanest reliable swim, though South Shore beaches (Waikiki, Diamond Head area) are well-flushed too. Windward Oʻahu (Kāneʻohe Bay) tests higher due to multiple stream inputs. Kauaʻi: South Shore (Po‘ipu) is the cleanest. North Shore (Hanalei, Tunnels) tests dramatically dirtier — the latest BWTF reading at Tunnels Stream Makua was 2,603 MPN/100mL (20× the 130 BAV).

Question 3

Is the wet side always worse for swimming?

Accepted Answer

Not always — but consistently after rain. Windward coasts have stronger ocean flushing from trade-wind-driven currents, which clears bacteria faster than the same volume of contamination on a sheltered leeward bay. So on a long dry stretch, windward beaches can test fine. The problem is that windward Hawaiʻi gets more rain, more often. The wet side has a higher BASELINE contamination from year-round stream flow plus a higher PEAK contamination after every rain event. Leeward dry coasts get fewer events and clear faster between them. If you have any rainfall in your trip window, the leeward side is the safer plan.

Question 4

Does cesspool density correlate with windward Hawaiʻi?

Accepted Answer

Yes. Hawaiʻi has roughly 88,000 active cesspools — direct discharge of untreated sewage into the soil. Cesspools are concentrated in older communities, many of which are on windward valleys (the historically populated agricultural areas of windward Oʻahu, the East Maui Hāna coast, and rural Kauaʻi). Cesspool effluent leaches into groundwater, into streams, into the ocean. The compounding effect: windward coasts get more rain (carrying contamination), they have more streams (concentrating contamination), AND they have more cesspools (a continuous bacteria source independent of rain). State law mandates cesspool conversion by 2050; meanwhile, windward beach water-quality reflects the legacy infrastructure.

Question 5

Where should I stay in Hawaii if water quality is a priority?

Accepted Answer

For the most reliable swim conditions, base your trip on the leeward (dry) side. Top picks by island: Big Island Kohala Coast (Mauna Lani, Mauna Kea Resort area, Hapuna) — the single cleanest stretch in Hawaiʻi. South Maui (Wailea or Kihei). West Maui (Kāʻanapali or Kapalua) — also leeward, slightly more rainfall than South Maui but still dramatically drier than Hāna. Oʻahu Ko Olina lagoons — protected swim pools with consistently low bacteria readings. Kauaʻi Po‘ipu — South Shore, leeward, the driest stretch on Kauaʻi. If you must base on the windward/wet side (e.g., visiting Hāna or Hanalei), schedule swimming for late in your stay so any rain has time to clear before you enter the water, and always verify current advisories.

Question 6

What does BWTF data show about leeward vs windward Hawaii?

Accepted Answer

The Surfrider Blue Water Task Force tests over 100 Hawaiʻi sites across 3 chapter labs. The geographic pattern in BWTF readings is striking. Leeward-coast sites consistently read at 10–30 MPN/100mL (well within the 130 BAV threshold): Magic Island, Wailupe, Makalei, Kaʻalāwai (south Oʻahu); Kalepolepo, Pohaku Park, Shark Pit (leeward Maui); Kukuiʻula Bay (south Kauaʻi). Windward-coast and stream-mouth sites consistently exceed BAV: Tunnels Stream Makua at 2,603 MPN, Hanalei River at 1,112 MPN, Anahola Stream Mouth at 2,851 MPN (windward Kauaʻi). The single highest reading in BWTF Hawaiʻi data — 6,586 MPN at Kaupuni Stream — is on the windward-influenced Waiʻanae coast where storm runoff overlaps with cesspool clusters.

Coast	Beach	Recent reading	Status
Leeward	Hāpuna Beach	0/30 days under advisory	Reliably clean
Leeward	Anaehoʻomalu Bay (A-Bay)	0/30 days under advisory	Reliably clean
Leeward	Mauna Kea Beach	0/30 days under advisory	Reliably clean
Mid-coast	Magic Sands	0/30 days under advisory	Clean (West Hawaiʻi side)
Windward	Honoliʻi Beach (Hilo)	0/30 days but Hilo gets frequent island-wide BWAs after rain	Rain-sensitive

Coast	Beach	Recent reading	Status
Leeward (S)	Wailea Beach	9/30 days under advisory (Mar-Apr storm cycle)	Generally clean
Leeward (S)	Kalepolepo Beach Park	10 MPN BWTF	Within BAV
Leeward (W)	Kāʻanapali Beach	9/30 days under advisory	Generally clean
Leeward (W)	Shark Pit (Polanui-Uhailio)	10 MPN BWTF	Within BAV
N Shore	Pāʻia Bay	10 MPN BWTF	Within BAV (open exposure)
Windward (E)	Hāna Bay	213 MPN BWTF	Exceeds BAV
Windward (E)	Kōki Beach (rivermouth)	183 MPN BWTF + dangerous shorebreak	Exceeds BAV
Windward (E)	Waioka (Venus Pool)	448 MPN BWTF + closed access	Exceeds BAV

Coast	Beach	Recent reading	Status
Leeward (W)	Ko Olina Lagoons	22/30 days during recent island-wide BWA, 0 otherwise	Resort-grade clean (lagoon design)
Leeward (S)	Magic Island	31 MPN BWTF	Within BAV
South	Wailupe Beach Park	20 MPN BWTF	Within BAV
South	Kaʻalāwai (Black Point)	10 MPN BWTF	Within BAV
North	Sunset Beach	28/30 days under recent BWA cycle	Rain-sensitive
North	Haleʻiwa Beach Park	30/30 days under advisory	Persistent issues
Windward	South Kaneohe Bay	145 MPN BWTF	Exceeds BAV
Windward	Punaluʻu Beach Park	96 MPN BWTF	Within BAV (high-end)
Stream	Kaupuni Stream (Waiʻanae)	6,586 MPN BWTF	51× the BAV — highest in HI

Coast	Beach	Recent reading	Status
Leeward (S)	Po‘ipu Beach Park	22/30 recent (Kona Low cycle)	Generally clean
Leeward (S)	Kukuiʻula Bay	20 MPN BWTF	Within BAV
Leeward (S)	Brennecke's Beach	22/30 recent	Generally clean
East	Lydgate Beach	20–41 MPN BWTF (2 sample points)	Within BAV (man-made pool)
Windward (N)	Hanalei Bay	10 MPN at the bay center, 1,112 MPN at Hanalei River mouth	Bay clean / River-mouth exceeds BAV
Windward (N)	Tunnels Beach	22/30 days under advisory; Tunnels Stream Makua 2,603 MPN	Exceeds BAV
East	Anahola Stream Mouth	2,851 MPN BWTF	22× BAV

Dry Side vs Wet Side Hawaii

Hawaiʻi (Big Island)

Maui

Oʻahu

Kauaʻi