OPINION OF THE COURT
ROBERTS, Justice.
This is a case of first impression. At issue is the construction and constitutionality of Section 316 of The Clean Streams Law, Act of July 31, 1970, P.L. 653, § 12, amending 35 P.S. § 691.316. Section 316 provides in relevant part:
“Whenever the [Department of Environmental Resources] finds that pollution or a danger of pollution is [225]*225resulting from a condition which exists on land in the Commonwealth the [Department] may order the landowner or occupier to correct the condition in a manner satisfactory to the [Department] . . . .”1
The principal questions presented by this appeal are whether Section 316 authorizes the Department of Environmental Resources to remedy water pollution resulting from conditions other than mine drainage, and whether Section 316 is a constitutional exercise of the Legislature’s police power. Like the Environmental Hearing Board and the Commonwealth Court, we conclude that both questions must be answered in the affirmative. Accordingly, we affirm the order of the Commonwealth Court affirming the order of the Environmental Hearing Board.
I
The dispute in this case concerns a parcel of land in Delaware County owned by appellants Clifford and Virginia Rogers and leased in part by appellant National Wood Preservers, Inc.2 The Rogers have owned this parcel since 1942. In 1947 they leased it to Samuel T. Jacoby and C. David Jacobs, who then assigned the lease to National Wood Preservers, Inc. This company, all of whose stock was owned by Jacoby, conducted a wood preservative business on the parcel between 1947 and 1963. National Wood Preservers, Inc. used a chemical called pentachlorophenol, “a toxic substance . . . lethal to acquatic organisms in certain [226]*226concentrations.”3 In the course of its operations, National Wood Preservers, Inc. disposed of waste liquids containing pentachlorophenol, by discharging them into a well which drained into the groundwaters running beneath the premises. In 1963 Jacoby sold his shares in National Wood Preservers, Inc. to the Goldsteins.4 The Goldsteins have continued to operate National Wood Preservers, Inc. as a wood preservative business on the land in question.
On June 12, 1972, in response to numerous complaints, the Department of Environmental Resources (DER) initiated its investigation of an oily substance found in Naylors Run, a stream flowing near appellants’ tract. On the basis of this investigation, which included the collection and analysis of numerous water samples from the area around Naylors Run, DER determined that the groundwaters of that tract contain a polluting substance of pentachlorophenol and fuel oil. In 1973, pursuant to Section 316 of The Clean Streams Law, DER issued orders to appellants Rogers and appellant National Wood Preservers, Inc. to abate this harmful condition.5
Appellants appealed to the Environmental Hearing Board. The Board consolidated the appeals and conducted extensive hearings over a thirteen day period. The Board found inter alia that the pentachlorophenol, which was mixed with fuel oil in the waters of the Commonwealth, constitutes pollution within the meaning of Section 316, see 35 P.S. § 691.1, and that the major amount of this substance appears to be pooled under the surface of the property owned by Rogers and leased in part by National Wood Preservers, Inc. The [227]*227Board therefore ordered appellants, under the supervision of DER, first to conduct drilling and water sampling to determine the precise amount and dispersion of the pollutant, and then to remove it.6 The Board also found that the program for removal of the pollutant outlined in its findings and order was feasible.7 Appellants filed timely appeals to the Commonwealth Court. That court consolidated the appeals and unanimously affirmed the orders of the Environmental Hearing Board as to appellants. This Court granted allowance of appeal, also consolidating the appeals.8
II
Appellants’ first contention is that the Legislature, in enacting Section 316, intended to permit the Department of [228]*228Environmental Resources to order a landowner or occupier to correct a condition which results in pollution or the danger of pollution only if the condition were caused by mining operations. Like the Environmental Hearing Board and the Commonwealth Court, we reject this contention.
As the previously quoted portion of Section 316 makes evident, the Legislature has clearly and unambiguously authorized DER to require the correction of water pollution-causing conditions without regard to the source of the pollution. Indeed, the caption of Section 316, “Responsibilities of landowners and land occupiers,” suggests the section’s breadth. See 1 Pa.C.S.A. § 1924 (“The headings prefixed to sections and other divisions of a statute shall not be considéred to control but may be used to aid in the construction thereof.”); compare 35 P.S. § 691.2 B. The Legislature has instructed that “[w]hen the words of a statute are clear and free from all ambiguity, the letter of it is not to be disregarded under the pretext of pursuing its spirit.” 1 Pa.C.S.A. § 1921(b). In accord with this legislative mandate, this Court must conclude that Section 316 authorizes DER’s actions here.
Our reading of Section 316 is indeed entirely in harmony with, and fully in accord with, the Legislature’s objective, see 1 Pa.C.S.A. § 1921(c), as expressly set forth in Section 4 of the Act, 35 P.S. § 691.4:
“(1) Clean, unpolluted streams are absolutely essential if Pennsylvania is to attract new manufacturing industries and to develop Pennsylvania’s full share of the tourist industry;
(2) Clean, unpolluted water is absolutely essential if Pennsylvanians are to have adequate out of door recreational facilities in the decades ahead;
(3) It is the objective of the Clean Streams Law not only to prevent further pollution of the waters of the Commonwealth, but also to reclaim and restore to a clean, unpolluted condition every stream in Pennsylvania that is presently polluted;
[229]*229(4) The prevention and elimination of water pollution is recognized as being directly related to the economic future of the Commonwealth; and
(5) The achievement of the objective herein set forth requires a comprehensive program of watershed management and control.”
From this “Declaration of Policy,” enacted in 1970 along with the portion of Section 316 relevant here, it is clear that the Legislature seeks to eliminate all water pollution, not only water pollution emanating from mines, and to “reclaim and restore” every polluted stream. Thus any contrary or narrower reading of Section 316 would fundamentally undermine the efforts of DER to achieve these legislative objectives, as well as frustrate the Legislature’s fulfillment of its obligation under Article I, section 27 of the Pennsylvania Constitution:
“The people have a right to clean air, pure water, and to the preservation of the natural, scenic, historic and esthetic values of the environment. Pennsylvania’s public natural resources are the common property of all people, including generations yet to come. As trustee of these resources, the Commonwealth shall conserve and maintain them for the benefit of all the people.”
Nevertheless, appellants claim that Section 316 should be restricted. They cite provisions of The Clean Streams Law enacted in 1965, see Act of August 23, 1965, P.L. 372, five years before enactment of the relevant portion of Section 316, enacted in 1970. It is claimed that these provisions, including Section 4 setting forth the purpose of the Act, demonstrate a legislative concern for mine drainage pollution only. Appellants also cite another provision of Section 316, enacted in 1965 and inapposite here, which permits DER to “order such owner or occupier to allow a mine operator or other person or agency of the Commonwealth access to the land to take such action.” Additionally, they point to Section 316’s proximity to Section 315, enacted in 1965, a section concerned with mines.
[230]*230Our consideration of the history of The Clean Streams Law compels us to reject appellants’ limited view of the Legislature’s purpose in enacting the relevant portion of Section 316. The 1970 amendments to The Clean Streams Law remove any doubt as to the breadth of the statute’s purpose. Those amendments eliminate from the very 1965 provisions concerning the Act’s purpose, upon which appellants rely, any mention of mine-produced pollution.
In any case, even the 1965 provisions to which appellants refer lend no support to their contention. There is no doubt, of course, that until enactment of the 1965 amendments to The Clean Streams Law, the Legislature had not yet authorized a state agency to remedy the growing pollution problem posed by mine drainage.9 See generally Commonwealth v. Barnes & Tucker, 455 Pa. 392, 395-401, 319 A.2d 871, 873-76 (1974) (Barnes & Tucker I); Commonwealth v. Harmar Coal Co., 452 Pa. 77, 83-86, 306 A.2d 308, 312-13 (1973). The 1965 amendments were enacted in part to provide an administrative remedy for this problem.
It is equally clear, however, that the Legislature also enacted the 1965 amendments to combat water pollution generally. The dual purpose of these amendments becomes clear upon reviewing the “Findings & Declarations of Policy” enacted as part of the 1965 amendments, § 2. Half the findings concerned the problem posed by mine drainage, and the other half concerned the problems of water pollution generally. More important, the declarations of policy expressly stated:
[231]*231“It is the objective of the Clean Streams Law not only to prevent further pollution of the waters of the Commonwealth, but also to reclaim and restore to a clean, unpolluted condition every stream in Pennsylvania that is presently polluted . . . .”
This statement of purpose of The Clean Streams Law, never before expressly articulated by the Legislature, did not distinguish between sources of water pollution. Rather, all water pollution was proscribed. Thus even appellants’ own argument, turning as it does on the Act as it existed before enactment of the relevant portion of Section 316, must fail.
Ill
Appellants’ second contention is that Section 316 of The Clean Streams Law is an impermissible exercise of the police power, in violation of the Fourteenth Amendment of the United States Constitution and Article I, section 10 of the Pennsylvania Constitution. Again, we agree with the conclusion of the Environmental Hearing Board and the Commonwealth Court to the contrary.
The “police power” is one of the “most essential powers of government . . . .” Hadacheck v. Sebastian, 239 U.S. 394, 410, 36 S.Ct. 143, 145, 60 L.Ed. 348 (1915). It has been variously defined as the power “to promote the public health, morals or safety and the general well-being of the community,” Commonwealth v. Harmar Coal Co., supra, 452 Pa. at 92, 306 A.2d at 316; see DePaul v. Kauffman, 441 Pa. 386, 393, 272 A.2d 500, 504 (1971), or as “the inherent power of a body politic to enact and enforce laws for the promotion of the general welfare,” Commonwealth v. Barnes & Tucker, 472 Pa. 115, 123, 371 A.2d 461, 465 (1977) (Barnes & Tucker II), or as a power extending to “all the great public needs,” Day-Brite Lighting, Inc. v. Missouri, 342 U.S. 421, 424, 72 S.Ct. 405, 407, 96 L.Ed. 469 (1952).10 The [232]*232police power is fundamental because it enables “civil society” to respond in an appropriate and effective fashion to changing political, economic, and social circumstances, and thus to maintain its vitality and order. See, e. g., Mugler v. Kansas, 123 U.S. 623, 668, 8 S.Ct. 273, 301, 31 L.Ed. 205 (1887). “The police power of the state [must therefore be] . as comprehensive as the demands of society require under the circumstances.” Barnes & Tucker II, 472 Pa. at 126, 371 A.2d at 467. Of necessity, then, the police power is a broad and flexible power. See, e. g., Berman v. Parker, 348 U.S. 26, 32, 75 S.Ct. 98, 102, 99 L.Ed. 27 (1954); Euclid v. Ambler Realty Co., 272 U.S. 365, 386-90, 47 S.Ct. 114, 118-19, 71 L.Ed. 303 (1926). Indeed, it is the state’s least limitable power. See Hadacheck v. Sebastian, supra at 410, 36 S.Ct. at 145.
A state power as broad as the police power inevitably gives rise to tensions between the state and holders of property. Although the police power “may, indeed, seem harsh in its exercise, [and] usually is on some individual, . the imperative necessity for its existence precludes any limitation upon it when not exerted arbitrarily.” Hadacheck v. Sebastian, supra at 410, 36 S.Ct. at 145; see Miller v. Board of Public Works, 195 Cal. 477, 484, 234 P. 381, 383 [233]*233(1925). Therefore as long as the Legislature exercises that power in a reasonable and nonarbitrary manner, the judiciary will not invalidate the enactment.11 See City of New Orleans v. Dukes, 427 U.S. 297, 303-04, 96 S.Ct. 2513, 2516-17, 49 L.Ed.2d 511 (1957); Nebbia v. New York, 291 U.S. 502, 525, 54 S.Ct. 505, 510-11, 78 L.Ed. 940 (1934); Tosto v. Pennsylvania Nursing Home Loan Agency, 460 Pa. 1, 9, 331 A.2d 198, 201-02 (1975); accord Sproles v. Binford, 286 U.S. 374, 388, 52 S.Ct. 581, 585, 76 L.Ed. 1167 (1932) (even when the reasonableness of legislation is “fairly debatable,” the judiciary must refrain from disturbing it).
Review of the case law reveals that the police power has been constitutionally exercised in many ways over innumerable types of property. See generally Penn Central Transportation Co. v. New York, 438 U.S. 104, 123-28, 98 S.Ct. 2646, 2658-62, 57 L.Ed.2d 631 (1978). Of relevance here, legislation protecting state water resources has been held to be within the scope of the police power. See, e. g., Hudson Water Co. v. McCarter, 209 U.S. 349, 28 S.Ct. 529, 52 L.Ed. 828 (1908); Barnes & Tucker Co. v. Pennsylvania, 434 U.S. 807, 98 S.Ct. 38, 54 L.Ed.2d 65 (1977), dismissing appeal for want of a substantial federal question, Barnes & Tucker II, supra; Harmar Coal Co. v. Pennsylvania, 415 U.S. 903, 94 S.Ct. 1395, 39 L.Ed.2d 460 (1974), dismissing appeal for want of a substantial federal question, Commonwealth v. Harmar Coal Co., supra; Commonwealth v. Emmers, 221 Pa. 298, 70 [234]*234A. 762 (1908).12 Moreover, the police power has been used to impose new costs on property owners. See, e. g., Atchison, T. & S. F. Ry. Co. v. Public Utilities Comm’n, 346 U.S. 346, 74 S.Ct. 92, 98 L.Ed. 51 (1953); Queenside Hills Realty Co. v. Saxl, 328 U.S. 80, 66 S.Ct. 850, 90 L.Ed. 1096 (1946); Erie R. Co. v. Board of Public Utility Comm’n, 254 U.S. 394, 41 S.Ct. 169, 65 L.Ed. 322 (1921).
In Lawton v. Steele, 152 U.S. 133, 14 S.Ct. 499, 38 L.Ed. 385 (1894), the Supreme Court articulated the standard to be used by courts in determining the validity under the United States Constitution of a state’s exercise of its police power.
“To justify the State in thus interposing its authority in behalf of the public, it must appear, — first, that the interests of the public . . . require such interference; and, second, that the means are reasonably necessary for the accomplishment of the purpose, and not unduly oppressive upon individuals.”
152 U.S. at 137, 14 S.Ct. at 501. This Court has adopted this standard in our assessment of regulatory legislation under the Pennsylvania Constitution. See Commonwealth v. Harmar Coal Co., supra, 452 Pa. at 93, 306 A.2d at 317; Barnes & Tucker I, supra, 455 Pa. at 418, 319 A.2d at 885; Barnes & Tucker II, supra, 472 Pa. at 123, 371 A.2d at 465.
It is fundamental to our jurisprudence that enactments of the Legislature are clothed with a presumption of constitutional validity, and that appellants, by claiming that an act is unconstitutional, carry a heavy burden of proof. See, e. g., United States v. Vuitch, 402 U.S. 62, 70, 91 S.Ct. 1294, 1298, 28 L.Ed.2d 601 (1971); In re William L., 477 Pa. 322, 329, 383 A.2d 1228, 1231 (1978); Tosto v. Pennsylvania Nursing Home Loan Agency, supra, 460 Pa. at 16, 331 A.2d at 205, quoting Daly v. Hemphill, 411 Pa. 263, 271, 191 A.2d 835, 840 (1963) (“Courts may not declare a statute unconstitutional ‘unless it clearly, palpably and plainly violates the Constitution.’ ”).
[235]*235Appellants do not contend that Section 316 fails to satisfy Lawton’s first prong. As stated above, Article I, section 27 of the Pennsylvania Constitution imposes a duty upon the Commonwealth to protect our environment. Indeed, maintenance of the environment is a fundamental objective of state power. As Justice Holmes wrote:
“[T]he state has an interest independent of and behind the titles of its citizens, in all the earth and air within its domain. . . . It is a fair and reasonable demand on the part of a sovereign that the air over its territory should not be polluted . . . , that the forests on its mountains, be they better or worse, and whatever domestic destruction they have suffered, should not be further destroyed or threatened . . . , that the crops and orchards on its hills should not be endangered . . . .”
Georgia v. Tennessee Copper Co., 206 U.S. 230, 237-38, 27 S.Ct. 618, 619, 51 L.Ed. 1038 (1907). The Legislature has long regulated the quality of the waters in the Commonwealth. See Purity of Waters Act, note 9, supra; cf. Commonwealth v. Locust Point Quarries, Inc., 483 Pa. 350, 358, 396 A.2d 1205, 1209 (1979) (“In sum, protection of air resources is a matter of highest priority in the Commonwealth.”). In addition, other state legislatures, as well as Congress, have made extensive efforts to remove the pollution from our nation’s waters.13
[236]*236We are also convinced that Section 316 and the orders promulgated thereunder satisfy the first part of Lawton’s “means” prong. Section 316’s authorization of DER to order a landowner or occupier to correct conditions on his land causing pollution or a danger of pollution is “reasonably necessary” for eliminating water pollution. Certainly the owner or occupier of land is well situated to remove harmful conditions from his land. Likewise, the Environmental Hearing Board’s abatement orders are “reasonably necessary” in light of the Board’s well supported findings that pollution exists under the land of appellants, and that removal of the pollutant is feasible.14
Appellants focus their constitutional attack upon Lawton’s third requirement that the means are not to be “unduly oppressive upon individuals.” Though it is impossible to define this requirement precisely, sée Goldblatt v. Hempstead, 369 U.S. 590, 594, 82 S.Ct. 987, 990, 8 L.Ed.2d 130 (1962); see generally J. L. Sax, Takings and the Police Power, 74 Yale L.J. 36 (1964), a recent opinion of the United States Supreme Court, Penn Central Transportation Co. v. New York, supra, identifies two general factors of significance in the judicial determination of whether governmental action is unduly oppressive. The first consideration is the economic impact of the regulation on the property holder. Id. at 124, 98 S.Ct. at 2659. Specifically, it is relevant to compare property values before and after the regulation, though such a consideration is by no means conclusive. Compare Euclid v. Ambler Realty Co., supra (regulation reducing property value by 75% held constitutional) and Hadacheck v. Sebastian, supra (regulation reducing property value by %Th% held constitutional) with Pennsylvania Coal [237]*237Co. v. Mahon, 260 U.S. 393, 43 S.Ct. 158, 67 L.Ed. 322 (1922) (regulation rendering property worthless held unconstitutional).
The second factor identified in Penn Central is the character of the governmental action. 438 U.S. at 124, 98 S.Ct. at 2659. The greater the extent to which governmental interference with property can be characterized as a physical intrusion, the more likely it is that such interference will be considered an unreasonable exercise of police power. Compare Goldblatt v. Hempstead, supra (regulation prohibiting property’s most beneficial use is valid exercise of police power) and Euclid v. Ambler Realty Co., supra (regulation prohibiting property’s most profitable use held valid exercise of police power) with Causby v. United States, 328 U.S. 256, 66 S.Ct. 1062, 90 L.Ed. 1206 (1946) (U.S. airplanes taking off and landing over property held to be a taking since U.S. is in effect “using” that property).
Appellants do not argue that Section 316 is “unduly oppressive” because of its economic impact or its interference with their use of the property. Rather, appellants advance the somewhat unique argument that Section 316 is unduly oppressive because it imposes liability upon appellants solely on the basis of their ownership or occupancy of the land in question.15 It is unconstitutional, argue appellants, for DER to issue a corrective order to a landowner or occupier absent a showing of the party’s responsibility for causing the polluting condition.16
[238]*238We disagree. First it is absolutely clear that the corrective orders here are based on much more than mere ownership or occupancy. As discussed above, they are based upon legislation designed to eliminate all water pollution, and the Environmental Hearing Board’s findings that a substance, determined to be pollution, resides under appellant’s land and can feasibly be removed. There is thus a reasonable and concrete basis for the corrective orders here.
It is also clear that the validity of an exercise of police power over land depends little upon the owner or occupier’s responsibility for causing the condition giving rise to the regulation.17 For example, in Penn Central Transportation Co. v. New York, supra, the New York City Landmarks Preservation Commission designated Grand Central Station a “landmark” as part of the City’s comprehensive program to preserve historic landmarks and districts. Such a designation meant that the terminal owner was required to maintain the exterior architecture of the terminal in good repair, that the terminal facade could not be altered without approval of the Commission, and that the terminal’s economic potential could not be fully developed by the construction of an office building atop the terminal. The Supreme Court held the City’s landmark designation to be constitutional. It [239]*239is clear from the facts in Penn Central that the terminal owner was in no way responsible for causing the terminal’s landmark status: the owner did not “cause” the condition, nor can he be viewed as being “at fault.” The Supreme Court disregarded the owner’s lack of responsibility, however, and found the City’s action constitutional.
Miller v. Schoene, 276 U.S. 272, 48 S.Ct. 246, 72 L.Ed. 568 (1928) also illustrates that a property holder’s responsibility for the condition to be regulated is not an important factor in assessing the validity of the regulation. In that case a state agency pursuant to statute ordered property owners to cut down a large number of ornamental red cedar trees growing on their property because the trees produced cedar rust fatal to apple trees cultivated on adjacent land. Though the statute permitted the owners to use the felled trees and recover the costs of removing the trees, it did not provide compensation for the value of the trees or the diminished value of the property. The Court nonetheless found the statute constitutional, holding that a state could properly make “a choice between the preservation of one class of property and that of the other.” Id. at 279, 48 S.Ct. at 247. Since the apple industry was important to the state, the Court concluded that the state had not acted unconstitutionally “by deciding upon the destruction of one class of property [without compensation] in order to save another, which, in the judgment of the legislature, is of greater value to the public.” Id. The facts in Miller suggest that the property holders could not have caused the infection of their red cedar trees. The holding and discussion in Miller reflect the Court’s conviction that such a consideration is of little import.18 See also Goldblatt v. [240]*240Hempstead, supra; Euclid v. Ambler Realty Co., supra; Hadacheck v. Sebastian, supra. In light of Penn Central, Miller, and the other cases cited, appellants have failed to persuade us that the Commonwealth has unconstitutionally exercised its police power.19
Orders of the Commonwealth Court at Nos. 66 and 67 January Term, 1979 are affirmed. Commonwealth’s appeals from Commonwealth Court’s orders at No. 68 January Term, 1979 are dismissed.
NIX, J., did not participate in the consideration or decision of this case.
FLAHERTY, J., filed a concurring opinion.